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Full text of "Weather science: an elementary introduction to meteorology"

WEATHER SCIENCE 




HIGH STRATUS AND CIRRO-STRATUS CLOUDS. 
(Photo by Capt. Wilson Barker, F.R.S.E., F.R.Met.Soc,, &c.) 



Frontispiece, 



WEATHER SCIENCE 

AN ELEMENTARY INTRODUCTION 

TO 

METEOROLOGY 



BY 



F. W. HENKEL, B.A. 

FELLOW OF THE ROYAL ASTRONOMICAL SOCIETY, MEMBER OF THE BRITISH 
ASTRONOMICAL ASSOCIATION, LATE DIRECTOR OF MARKREE OBSERVATORY, 
ETC., ETC. 




T. FISHER UNWIN 

LONDON LEIPSIC 

ADELPHI TERRACE INSELSTRASSE 20 

1911 



[All rights reserved.] 



PREFACE 

IN the preparation of this work I must acknow- 
ledge my indebtedness to previous writers on 
the subject, and to the admirable Text-books, 
both popular and scientific, that have appeared 
from time to time. Amongst these I may 
mention the volumes by Dr R. H. Scott and 
Mr Abercromby in the International Scientific 
Series, Dr Waldo's " Modern Meteorology," Dr 
Hann's " Meteorologie," my friend Mr Inwards' 
" Weather Lore," Flammarion's "I/ Atmosphere," 
Dr Archibald's and Mr G. F. Chambers' popular 
works, as well as the transactions and journals 
of Meteorological Societies which have been 
consulted and referred to in due course. My 
indebtedness is also great to the late Commander 
M. F. Maury's "Physical Geography of the Sea," 
to the Text-books of Professor S. P. Thompson 
and Dr R. W. Stewart on Electricity, to the late 
Dr C. A. Young's " General Astronomy," etc. 
For the illustrations I am indebted to the 

kindness of Captain Wilson Barker, of H.M.S. 

5 

261609 



6. '.: Preface 

training ship Worcester, who sent me seven 
photographs of typical clouds, taken by himself, 
one of which appears as the frontispiece ; whilst 
Mr Inwards has permitted the reproduction of 
his plate of cloud forms, as well as the illustra- 
tions of Greenwich and Kew Observatories, and 
the Temple of the Winds, Athens ; to Dr H. 
R. Mill, head of the British Rainfall Organisa- 
tion, I owe the Rainfall Map that appears in 
chapter iv. Messrs Casella, Pillischer and Short 
and Mason have also kindly supplied illustrations 
of some of the instruments, barometers, ther- 
mometers, etc., described in the chapters on 
instruments. To all of these gentlemen for 
their help in these and other ways, as well as 
to others, my thanks are due. 

In addition 1 wish to express my gratitude 
in especial to Dr H. R. Mill and Mr Inwards 
for their kindly advice and recommendations 
during the course of publication. 

F. W. HENKEL. 



CONTENTS 



CHAPTER I 

PAGE 

Introductory Ancient ideas The barometer as a ' ( weather 
glass " Dawn of modern meteorology Cyclones and 
anticyclones The earth : its size, shape, and motions 
The atmosphere 13 



CHAPTER II 

The barometer Mercurial barometer Glycerine barometer 
Fortin's barometer Adjustments Syphon barometer 
Fishery barometer Aneroid barometer Self-recording 
barometers Corrections to reading of barometer 
Weather variations with barometer Oscillations of 
pressure, diurnal and annual Extreme range Dove's 
rules Regions of high and low pressure Pressure at 
various altitudes above the surface . 46 



CHAPTER III 

The thermometer Scales : Fahrenheit, Centigrade, Reaumur, 
De L'Isle The hypsometer " Wet and dry bulb " 
hygrometers De Saussure's hair hygrometer Maximum 
and minimum thermometers Self- recording ther- 
mometers (thermographs) The Stevenson screen 
Glaisher screen Black bulb thermometers Ranges in 
temperature . . . . . . . . .71 

7 



8 Contents 

CHAPTER IV 

PAQB 

Rain gauges Sunshine recorders Wind instruments 

Anemometers Distribution of rainfall . . .89 

CHAPTER V 

\l Weather forecasts Types of circulation The seven funda- 
mental forms of isobars Cyclones Secondaries 
V-shaped depressions Cols Straight isobars Anti- 
cyclones Wedges Lower and upper winds . . .103 

CHAPTER VI 

More detailed description of course of circulation Weather 
in (1) cyclones, (2) anticyclones Special varieties 
Whirlwinds Tornadoes . . . . . .114 

CHAPTER VII 

Clouds Howard's nomenclature Cirrus Cirro-cumulus- 
Cumulus Stratus Cirro - stratus Cumulo - stratus 
Nimbus Heights of different kinds of clouds Motions 
of clouds and air currents Fogs, mists, etc. Dust 
particles or other nuclei necessary Cloud " prognostics " 
of weather changes 131 

CHAPTER VIII 

The winds Direction Compass and true bearings Relation 
to isobars Beaufort's scale Buy Ballot's law Trade 
winds General circulation Special winds : Fohn, 
scirocco, bora, mistral, land and sea breezes [Note 
on wind velocities and Beaufort's scale] . . . .148 

CHAPTER IX 

Miscellaneous phenomena Snow Hail Dove's theory 
Volta's theory Dew Fog Mist Rainbows Halos 
Coronae The mirage Blue of the sky Tints of sunrise 
and sunset Dust . . . . . . . .161 



Contents 9 

CHAPTER x 

PAGE 

Methods of heat transference Ocean currents The Gulf 
Stream Influence on temperature and other weather 
conditions Theories as to its origin, etc. The Humboldt 
or Peruvian Current Cooling effect China Current 
" Kuro Siwo '' Currents of the Indian Ocean Agulhas 
Current Polar Currents Ideas as to their causes . .181 

CHAPTER XI 

Atmospheric electricity Instruments Quadrant electro- 
meter Portable electrometer Thunderstorms Light- 
ning conductors St Elmo's fire Electric "Hum" 
The Aurora Magnetic storms Magnetic and true 
bearing 199 

CHAPTER XII 

Seasonal variations of weather Recurrent types Buchan's 
hot and cold periods Indian summer St Luke's summer 
St Martin's summer May and November cold spells, 
etc. Southerly, northerly, westerly, and easterly types 
( Abercromby) Southerly type Weather conditions 
Westerly type Northerly type Easterly type 
Distribution of pressure over the globe Polar cyclones 
The Doldrums 225 

CHAPTER XIII 

Observatories First order, second order, and third order 
stations Greenwich Observatory Self-recording instru- 
ments Kew Observatory Second order stations Their 
work Bi-daily observations Instruments Observations 
Morning series Evening series Forecasts of weather 
Storm warnings List of districts Daily and weekly 
weather reports [Note on metric and " English " 
measures] -Temple of the Winds, Athens . . . 239 

CHAPTER XIV 

Weather signs and portents of coming changes Sun Moon 
Clouds Cirrus Cirro-stratus Cirro-cumulus 



io Contents 

PAGE 

Stratus Cumulus Cumulo-stratus Colours of the 
sky Animals Birds 261 

CHAPTER XV 

Cycles of weather Bruckner's investigation Rainfall Wet 
and dry periods Oscillations of mean pressure and 
temperature Sun-spots and the weather Sun-spot 
" period": its irregular length The moon . . . 272 

CHAPTER XVI 

The seasons Heating action of the sun Zones Climate 
affected by proximity to sea Isothermal lines : their 
course across the globe Extremes of temperature 
Isabnormals Sea temperatures Influence of Gulf Stream, 
etc. Minimum temperature of sea 285 

CHAPTER XVII 

Upper air Balloon ascents Kites Isothermal layer 

Division of air into two regions, lower and upper . .311 

CHAPTER XVIII 

Practical applications " Signs" of rain The leech - 
Seasonal changes Migration of birds Plant " Phen- 
ology" Flowering of plants Fall of leaves Seasonal 
prevalence of diseases Salubrity of particular regions 
Ozone .' . .318 

INDEX 331 



LIST OF ILLUSTRATIONS 

HIGH STRATUS, AND CIRRO-STRATUS CLOUDS Frontispiece 

STANDARD BAROMETER. (FORTIN'S PATTERN) . page 50 

ANEROID . 54 

BAROGRAPH ... . 56 

WET AND DRY BULB HYGROMETER . . . ,, 76 

MAXIMUM THERMOMETER ) _ Q 

f /y 

MINIMUM THERMOMETER > 

STEVENSON SCREEN ,,83 

SOLAR RADIATION, BLACK BULB THERMOMETER 



84 

GRASS MINIMUM THERMOMETER 

THE HYGRODEIK . . . . . . 88 

RAIN GAUGE AND MEASURING GLASS . 90 

SUNSHINE RECORDER ,,93 

ANEMOMETER WITH WIND VANE . . . 95 
ROBINSON'S ANEMOMETER ... . 96 

MEAN ANNUAL RAINFALL MAP OF THE BRITISH 

ISLES to face page 102 

SELF-RECORDING RAIN GAUGE .... page 102 

CLOUD FORMS, BY COL. H. M. SAUNDERS . tofacepage 131 

CUMULUS AND HIGH STRATUS ) 

} . 134 

CIRRUS 



) 



SQUALL CUMULUS 
CUMULUS 

11 



12 List of Illustrations 

CIRRO-CUMULUS ^ 

> to face page 140 

GROUND FOG J 

QUADRANT ELECTROMETER . ... page 202 

GREENWICH OBSERVATORY (METEOROLOGICAL 

SECTION) to face page 239 

KEW OBSERVATORY (NATIONAL PHYSICAL 

LABORATORY) .... 243 

TEMPLE OF THE WINDS, ATHENS . ,,260 

MEAN ANNUAL ISOTHERMS FOR THE BRITISH ISLES 

(AFTER BUCHAN AND MILL) .... page 286 



WEATHER SCIENCE 
CHAPTER I 

INTRODUCTORY ANCIENT IDEAS THE BAROMETER AS A " WEATHER 

GLASS" DAWN OF MODERN METEOROLOGY CYCLONES AND 

ANTICYCLONES THE EARTH I ITS SIZE, SHAPE, AND MOTIONS 

THE ATMOSPHERE. 

EVERY one thinks that he or she knows some- 
thing about the weather, and its vagaries, real 
or assumed, are an unfailing subject of conver- 
sation, affording ample opportunities for the 
indulgence in the Englishman's privilege of 
grumbling. Yet, notwithstanding the multi- 
plicity of weather saws and sayings, the enor- 
mous mass of statistical details collected by 
the industry of countless observers in our own 
and other countries and published in ponderous 
tomes, whose covers no one ever opens, by Govern- 
mental departments and learned societies, it must 
be confessed that the science of the weather is 
as yet in its infancy. Very little has been done 
in the way of utilising " the dry masses of figures 
which accumulate year after year, and lie abso- 
lutely idle in yearly volumes, unread and un- 
studied," but still the game of accumulation goes 

13 A 



14 Weather Science 

on as merrily as ever. However, of recent years 
some attempts have been made to arrive at a 
better state of things ; our knowledge, not merely 
of the bottom of the "ocean of air" in which 
we live, but also of portions of its upper regions, 
has been increased by the judicious flying of 
kites with self - recording instruments of pre- 
cision, and much important information as to 
the distribution of temperature, moisture, etc., 
in those hitherto unknown heights has been 
obtained. The application of dynamical methods 
by Bezold and others, the development of theories 
of atmospheric circulation, the improvement of 
instruments and the establishment of regular 
meteorological observatories, not only in the 
lowlands but also in mountain regions, these and 
other things have all helped in throwing light 
upon some of the obscurer problems of the 
science. The complexity of the subject, how- 
ever, seems to give little hope that the ideal of 
deducing all weather changes from the knowledge 
of the variations in the amount and direction 
of the solar radiation, in their effect upon the 
mixture of air and water vapour of which our 
atmosphere consists, will be soon realised. 

For a long time to come Meteorology must 
be regarded as a science of pure observation, 
and our knowledge of it confined to generalisa- 
tions obtained with more or less exactness as 
the result of these observations. In early days, 
Meteorology (from the Greek ra /uerewpa, things 



Aristotle's Views 15 

above), included not only the study of the 
atmosphere, its clouds and weather changes, 
but also such things as comets and shooting 
stars (or meteors, as they are distinctively called), 
which, being now known to be extra-terrestrial 
phenomena, are removed to the domain of 
Astronomy. Accordingly we find in Ptolemy's 
"Almagest," the greatest work of the ancient 
Greek astronomy, no mention of comets at all, 
the latter being regarded as mere temporary 
exhalations or vapours from the ground drawn 
upwards, and burning when they reached the 
" region of fire." Aristotle, perhaps the most 
universal genius who ever lived, whose works 
and ideas on almost every subject were held in 
the utmost reverence till about two centuries 
ago, considered that the atmosphere is divided 
into three regions. The first region is that in 
which animals and plants live, supposed to be 
immovable like the earth on which it rests ; 
the second is an intermediate region, intensely 
cold ; the third region, contiguous to the region 
of fire, or the heavens, partakes of the diurnal 
motion of the latter. Vapours arising from the 
earth ascend to this region and are heated, 
engendering igneous meteors and comets. A 
characteristic feature of these ideas is the slender 
basis of fact upon which they rest. 

Where experiment or observations were few, 
or wanting altogether, the ancient writers seem 
to have found no difficulty in supplying the 



1 6 Weather Science 

deficiency by means of vivid imagination. A 
writer of authority made a definite statement, 
and his word passed for ages unimpeached, no 
attempt being made to ascertain whether the 
results of experiment agreed with the predic- 
tions of theory or not. In this way there have 
arisen fancies as to the connection between the 
" changes " of the moon and weather change, still 
widely believed in, though absolutely without 
foundation in fact ; others, after the spirit of the 
old astrologers, have brought in the planets, the 
infinitesimal variations in the sun's light and 
heat accompanying the greater or less spottedness 
of his surface, etc., as efficient causes. All such 
speculations, by attributing specific and different 
actions in different regions of the earth, bring 
their own refutation. Whatever influence, for 
instance, a sun spot may have upon terrestrial 
conditions, it is difficult to believe that it can 
cause extra heat in one region, deficient rain- 
fall in another, and specially fine weather in a 
third. So far we may be satisfied to look to 
nearer causes for such variations, and it must 
be confessed that the complexity of the subject 
is such that we can hope only for slow progress 
in our knowledge in this respect. 

The science of weather predictions, and the 
results obtained, will be alluded to in the course 
of this work in further detail. So far, however, 
greater success has been obtained in average 
results than in furnishing information which is 



Napoleon and Laplace 17 

available for any specific short interval, such as 
the total rainfall on a given day, or the actual 
temperature at a given place. 

An instance of the uselessness of average results 
as to mean temperature and other weather con- 
ditions in enabling predictions to be made for 
any specific future occasion is given by Mr 
Abercromby in his well - known book on the 
" Weather." Nearly a hundred years ago, 
Napoleon, on the eve of his invasion of Russia, 
requested Laplace to calculate when the cold 
set in severely over that country. The latter 
found that on the average it did not set in 
hard until January. " Napoleon made his plans 
accordingly, a sharp spell of cold came on in 
December, and the army was lost." In a similar 
manner we have found by long continued series 
of observations the mean height of the baro- 
meter for every day of the year and almost for 
every hour of the day at most inhabited places 
on the earth's surface, but this gives no informa- 
tion whatever as to what will be the actual 
height at any particular moment, in fact this 
is almost bound to be either higher or lower 
than the mean value. 

The old weather prognostics, many of which 
we inherit from classical days, and some from 
yet earlier periods, have been handed down into 
all European languages, and are still of great 
value. Most of the saws and sayings relating 
to the approach of rain, perhaps the most 



1 8 Weather Science 

important point which concerns all, young and 
old, rich and poor alike, are well known, and 
their general accuracy has been often testified 
to, though it is only within recent years that 
some reasons for their verification as well as 
explanations of their occasional failure have been 
forthcoming. 

The invention of the barometer nearly three 
centuries ago, and the consequent knowledge 
of the variations in atmospheric pressure, led to 
the discovery that, on the whole, the mercury 
fell for rain and windy w r eather, and rose for 
fine, being generally low when the conditions 
were unfavourable, and high when fine settled 
weather prevailed. Hence arose the designa- 
tion of this instrument as a "weather glass," 
and the conventional but incorrect notation still 
to be met with even now, some makers alleging 
that the public will not buy barometers without 
these indications. Yet a very short series of 
observations will suffice to show that rain some- 
times falls with a high and rising barometer, 
and there are frequent occasions of good weather 
when the "glass" is low. Of more scientific 
value are the instructions given in the well- 
known Fitzroy or " fishery " form of barometer, 
these embody the results of careful observations. 

In our latitudes the barometer usually falls 
with S.E., S., or S.W. winds, and rises with 
W., N.W., and N. ones. Thus most rain 
coming with southerly and south - westerly 



Synoptic Charts 19 

winds, owing to their having blown over the 
ocean before reaching us, this explains the 
ordinary statement, but since rain sometimes 
comes with a northerly wind also, the rise of 
the barometer often deceives those who expect 
fair weather from the latter circumstance. 

The method of synoptic charts, giving the 
conditions of temperature and pressure, direction 
and force of the winds, etc., over large areas of 
the earth's surface (as illustrated by the daily 
diagrams published by the Meteorological offices 
of this and other countries which are reproduced 
in the newspapers), constituted a distinct advance 
in the science of weather predicting. Tele- 
graphic information from a number of stations 
is transferred to an outline map. Lines are 
drawn through all places having the same 
barometric height, these isobars? as they are 
called, being usually marked at intervals of 
Y 1 ^ inch or 2 mm. (5 mm. = *2 inch very nearly) of 
the mercurial barometer, and from the nature 
and present arrangement of these lines important 
information as to impending changes in weather 
may be easily gathered. 

The direction and force of the wind at various 
places is marked, as also the temperature, at a 
given time, lines drawn through places having 
the same temperature being known as isothermal 
lines, or simply "isotherms." A general rule 

1 All places in the neighbourhood where the pressure is for the 
moment the same are on the same isobar (uros, equal ; fiapvs, heavy). 



2O Weather Science 

governing the relation between the direction 
of the wind and the position of the isobars was 
enunciated by Buys Ballot in 1860, and is 
accordingly known by his name. "If one 
stands with his back to the wind, the baro- 
meter will be lower on his left hand than 
on his right." Thus when the barometer is 
higher to the north than to the south, the 
wind will be east, when it is higher to the east 
than to the west the wind will be southerly, 
and so on. It is evident, however, that this 
knowledge can only be possessed by an observer 
who knows the conditions at other stations than 
his own. The movement of air being due to 
the difference of pressure at different regions, 
it follows that where the "gradient" is small, 
or the isobars are far apart, the wind will be 
in general slight ; when, on the other hand, the 
isobars are close together, there will be more 
wind, and a "serious storm may be expected 
when there is a difference of half an inch of 
pressure between two neighbouring stations." 
Speaking generally, it may be said that the 
force of the wind will not exceed that of a 
"strong" or "fresh" breeze, unless the gradient 
exceeds 0*02 inch for a distance of 15 geo- 
graphical miles, approximately equal to 2 mm. 
per degree of latitude (Abercromby). 

Seven different forms of isobars are usually 
distinguished, of which two, the " cyclone " and 
the "anticyclone," are the most familiar to 



Cyclones and Anticyclones 21 

ordinary readers. The former is a region of 
low pressure, surrounded by more or less nearly 
circular isobars, hence the name (from the Greek 
KVK\O$, a circle). These, however, usually have 
the form of somewhat elongated ovals, and are 
by no means concentric. At or near the centre 
of the innermost oval is the point of lowest 
pressure, spoken of as the centre, or "eye," of 
the cyclone. Generally speaking, cyclones are 
in fairly rapid motion, in this country usually 
from S.W. towards N.E., and the wind rotates 
round the centre in the positive or " counter- 
clockwise" direction (that contrary to the 
direction in which the hands of a clock move), 
its intensity depending on the closeness of 
the isobars, being nearly proportional thereto. 
Occasionally cyclones are stationary, break up 
or move in an unexpected direction hence one 
cause of the failure of predictions founded on 
the supposition that they will follow their usual 
course. The cyclones which cause the much 
dreaded tropical storms are of the same nature, 
and differ principally from those we experience 
in these latitudes, in the much greater varia- 
tions of pressure within a short distance and 
the consequent greater violence of the wind. 
In our country the velocity of a cyclone centre 
may vary from 10 to 70 miles per hour, and, 
as we have said, its motion is usually eastward, 
though a few move in the westerly direction, 
whilst others are stationary. The air to the front 



22 Weather Science 

is usually warmer than that in the rear, and thus 
gives rise to the well-known close, "muggy" 
sensation, that in the rear is characterised by 
a brisk, exhilarating feeling. The anticyclone, 
on the other hand, is a region of high pressure, 
surrounded by isobars usually much further 
apart than those of a cyclone, and consequently 
covering a larger area. Light winds circulating 
in the " clockwise " direction ; or calms, prevail, 
and sometimes for days or even weeks there 
is practically no motion of the system at all, 
until it breaks up or moves slowly on. The 
ordinary features of weather are a clear sky 
and dry air, varied sometimes by a few light 
showers, in summer a hot sun, in winter fog 
and frost. The other forms of isobars will be 
dealt with more in detail later on in the course 
of this work (chap. v.). 

The earth, " our common mother," the stand- 
point from which we view the rest of the 
universe, is one of a number of more or less 
spherical bodies of varying sizes moving round 
a centre, known as the planets of the solar 
system. It is the third in order of distance 
of the planets moving round the central 
body, and its motions may be roughly divided 
into two principal ones, the diurnal rotation 
on its axis, causing the succession of day and 
night, and the annual motion round the sun, 
causing (in our latitudes) the phenomena of the 
seasons. In shape the earth is not strictly of 



Size and Shape of Earth 23 

the form of any regular geometrical figure, but 
approximates to an oblate spheroid of revolu- 
tion, i.e., a figure like a sphere (the distance of 
every point of whose surface from the centre is 
the same), but with a greatest and least diameter, 
round the latter of which it turns once in 
twenty-four (sidereal) hours. The least diameter 
(polar) is as nearly as possible 7,899 miles, the 
greatest (equatorial) diameter is 7,925 miles, so 
that the deviation from exact sphericity, or the 
polar flattening, as it is sometimes called, is not 
great. Neither the elevation of mountains, nor 
the depression of the sea bottom, nor even the 
polar flattening, bear any considerable pro- 
portion to the whole size of the earth, the 
highest mountain never rising more than 6 
miles above the general surface, whilst the 
deepest seas have less than that distance from 
surface to bottom, and the difference between 
equatorial and polar diameters (25 miles) is but 
a small fraction of either, so that a model made 
accurately in wood would be undistinguishable 
by the eye from an exact sphere. 

The diurnal rotation on its axis causes a point 
on the Equator (circumference 7,925 x 3-1416 
miles = 25,000 miles nearly) to move over more 
than 1,000 miles per hour, points in higher 
latitudes having slower motions, till at the 
Poles there is no rotation at all. 

In the latitude of London (51 J N.) the speed 
of rotation is about 600 miles per hour, this 



24 Weather Science 

varying directly as the cosine of the latitude, 
being at its maximum on latitude (equator) 
and zero at latitude 90 (poles, where cos 
90 = 0). The rotation of the earth on its axis, 
besides causing the succession of day and night, 
rising and setting of the stars, etc.. has a most 
important meteorological effect in causing the 
phenomena of the "trade" and "anti-trade" 
winds, the deviation of air currents produced 
during their movement from places moving 
more quickly to others of slower rotation, and 
vice versa, which will be dealt with later on in 
the course of this work. 

In addition to this motion of rotation the 
whole earth has a progressive motion round the 
sun, completing one circuit in the course of a 
year, moving in a path which is approximately 
circular (in reality slightly oval), and which 
lies in one plane (the ecliptic), which makes an 
angle of about 23| with the perpendicular to 
the axis of rotation (the Equator) ; or in other 
words, the inclination of the Equator to the 
ecliptic is this angle, whose mean value on 
1st January 1910 is given in the Nautical 
Almanac as 23 27' 3'58", diminishing at the 
rate of 0*468" per annum. These planes inter- 
sect in two points, called technically the first 
point of Aries and the first point of Libra, the 
sun appearing from the earth to be in the former 
on 21st March, when his longitude is said to 
be zero, and in the latter on 23rd September 



Annual Motion 25 

(approximately, varying slightly through the 
calendar arrangement of leap year, and the 
slightly unequal apparent motion of the sun), 
or perhaps, since it is the earth that moves and 
not the sun, it would be more correct to say 
that the earth is in "Libra" on 21st March 
and in "Aries" on 23rd September. 1 

Since, after all, all motion is in reality relative, 
it is not only convenient, but, so far as we are 
concerned, correct, to use the current language. 
Thus between 21st March and 23rd September 
the sun's position is on one side (north) of the 
equinox, whilst for the rest of the year he is 
on the other side (south) of that plane. When 
in the celestial Equator, which we may regard 
as the indefinite prolongation of the plane of 
the terrestrial Equator to the heavens, the sun 
is everywhere above the horizon for twelve 
hours, and for an equal time of day below it, 
and these times are consequently called the 
equinoxes (Latin : equus, equal ; nox, night), day 
and night being equal. After this, from 21st 
March to 21st June, the sun's distance from 
the Equator northwards becomes greater and 
greater; the "days" in our latitude, or times 

1 The signs Aries and Libra are now distinct from the constella- 
tions of those names, and are now approximately situated in the 
preceding constellations Pisces and Virgo. The " first point of 
Aries" is not very near any bright star, but not very far from 
the fourth magnitude star w Piscium ; the " first point of Libra " 
is between the double star p Virginis and the star 77 of that con- 
stellation, both of the third magnitude. Both these points, how- 
ever, are slowly changing in position owing to " Precession of the 
Equinoxes." 



26 Weather Science 

during which he is above the horizon, become 
longer and longer; the "nights," or periods 
when he is below, becoming correspondingly 
shorter. After this, reaching his greatest dis- 
tance north of the plane of the Equator ("first 
point of Cancer "), his distance from that plane 
gradually diminishes, till on 23rd September he 
is again in the celestial Equator. The days and 
nights are once more equal, the former having 
been gradually growing shorter and the latter 
longer from 21st June to 23rd September. 
After this time the sun's apparent motion carries 
him south of the Equator; the days are now 
shorter than the nights, the former continue de- 
creasing, the latter increasing, till the extreme 
values, both for the sun's angular distance (south) 
and the length of these periods, are attained on 
21st December (shortest day in our hemisphere). 
The varied phenomena of the seasons and the 
different altitudes of the sun, relative length 
of day and night in different parts of the globe, 
are fully explained in most geographical and 
astronomical works, so that we need not go 
much into detail here. It will suffice to say 
that since the days are longer, and the altitude 
of the sun greater in summer than in winter, 
we have here the primary cause of the differ- 
ence of temperature between those seasons ; 
whilst in spring and autumn both the length 
of the day and the altitude of the sun have 
intermediate values, and, as is well known, these 



Climates and Zones 27 

seasons are colder than summer, and (in general) 
warmer than winter with us. Thus the first 
and most general cause of the difference in 
temperature at different times of the year in our 
country is roughly indicated, whilst the differ- 
ence between the temperature in our latitudes 
and that experienced in " hot countries " nearer 
the Equator is primarily due to the more nearly 
vertical position of the sun in the latter regions. 
As one writer puts it, the same amount of heat 
is spread over a larger area, and so each spot 
receives less. 

At all places between latitudes 23^ N. and 
23^ S., which zone, bisected by the terrestrial 
equator, is known as the Torrid Zone, the 
limiting latitudes, 23^ N. and 23^ S. being 
known as the Tropics of Cancer and Capricorn 
respectively, the sun is in the zenith or vertical 
at some time or other during the year, and its 
altitude, when highest, at noon is never less 
than 90 47 = 43, whereas its greatest altitude 
above the horizon in the latitude of London 
(51), though 62 on 21st June, is less than 
this on every other day of the year, being only 
15 on 21st December. Everywhere throughout 
the Torrid Zone the length of the time the 
sun is above the horizon never varies greatly 
from twelve hours, the night being of about 
the same length. On the Equator itself the 
days and nights are always (theoretically) of 
equal length (neglecting the effect of refraction 



28 Weather Science 

by the atmosphere, which is always to slightly 
extend the former at the expense of the latter, 
and of which we shall soon have to speak), 
but further north and south there is a slight 
variation. 

At the Tropic of Cancer (23^), for example, 
the sun is vertical on 21st June, when he is 
said technically to enter the sign of that name, 
and the length of the day has then its greatest 
value, thirteen and a half hours, whilst the night 
is only ten and a half hours long. Conversely, 
on 21st December the sun has his least altitude 
at noon, only 43, and the day is only ten and a 
half hours long, the night being then thirteen 
and a half hours long. Between latitudes 23 -J- 
N. and S. and latitudes 66^ N. and S. respec- 
tively, the sun's greatest altitude varies from 90 
(when he is in the zenith), to nothing (when he is 
on the horizon), and the length of the day under- 
goes corresponding variation. Everywhere within 
these zones, the " Temperate Zones," he rises and 
sets at least once every day, though, theoreti- 
cally, at the summer solstice he barely touches 
the horizon at midnight, for latitude 66^, whilst 
at the winter solstice he scarcely rises at all. 

Between latitudes 66^ N. and S. and the 
respective Poles the " Frigid Zones " the sun 
has never an altitude greater than 47, about 
half-way between the horizon and the point 
overhead, so that his rays always fall more or 
less obliquely; and whilst at one time of the 



Phenomena at the Poles 29 

year (during summer of each hemisphere respec- 
tively) he is visible near the horizon at the 
time that would be midnight in lower lati- 
tudes, and the phenomenon of the "Midnight 
Sun" is seen, at another time he does not rise 
at all, and perpetual night, lasting for a longer 
or shorter period (during the winter of each 
hemisphere), arises. At the Poles the sun is 
alternately visible and invisible for about six 
months at a time (but see " Refraction " by the 
atmosphere, infra), but his greatest altitude 
never exceeds 23^, or its value at noon on 
8th February in the latitude of London, so 
that his rays at no time fall more than very 
obliquely, though his long continuance above 
the horizon will help to raise the temperature 
above what might otherwise be expected. The 
phenomena at either Pole will be somewhat 
different from anywhere else on the earth, so 
that the measurement of the sun's apparent altitude 
will afford a certain test as to whether the observer 
is, or is not, in latitude 90. If, now the sun's 
altitude be carefully measured when he is in 
one part of the sky, and again twelve hours 
later, the corrected readings, after allowance for 
refraction and instrumental errors, differ only 
by the amount of the sun's change of declina- 
tion during that interval, as given by the 
Nautical Almanac, then the observer may be 
confident that he is very close indeed to the 
position of the terrestrial Pole. 



30 Weather Science 

Though, as we have just stated, the primary 
cause of the different temperatures prevailing in 
different parts of the earth is to be looked for 
in the unequal amount of heat directly received 
from the sun, yet the relative distribution of land 
and water cause the actual values to be very 
different from what would otherwise be the 
case. Water having a greater specific heat 
than almost any other substance (that is, more 
heat is required to raise its temperature by a 
given amount than is the case for land surfaces, 
and more heat is given out by it in cooling 
through the same degrees of temperature than do 
land masses), the heat of summer is mitigated 
and the cold of winter moderated by the presence 
of large masses of water, for the lands adjoin- 
ing them hence arises the difference between 
"insular" and "continental" climates. Water 
covering ^ths of the earth's surface, whilst of 
the ^ths surface uncovered by water the greater 
part lies in the Northern Hemisphere, we find 
very considerable differences in temperature 
between points in corresponding latitudes, which 
receive almost exactly the same amount of sun 
heat, directly. As an equally important cause 
may be mentioned the existence of warm and 
cold currents, such as the Gulf Stream flowing 
from the Central Atlantic towards Western 
Europe, a warm current conveying some of the 
heat received by the tropical regions to the 
latter, the " Humboldt current " of the Southern 



The Atmosphere 31 

Pacific and "Polar currents" helping to cool 
the lands towards which they flow. 

But the earth is not merely a mass of land 
and water, otherwise neither man himself nor 
the science of meteorology could exist. Above 
and around; it, to an unknown height, extends 
the atmosphere or " ocean of air," a gaseous 
envelope. The main constituents of this atmos- 
phere are the gases oxygen, nitrogen, argon and 
carbon dioxide, with a variable amount of 
water vapour, and small traces of ammonia, 
nitric acid, etc. The density of this atmosphere 
is greatest at the earth's surface, and rapidly 
decreases as we go upward, but no certain 
limits can be assigned beyond which we can 
positively assert that there is no air at all, 
though at a height of above 100 miles from the 
surface the quantity of air must be very small 
indeed. Being a gas, or rather a mechanical 
mixture of a number of gases whose relative 
proportions vary very slightly at different times 
and places, the atmosphere possesses elasticity, 
exerts pressure, and is easily affected by changes 
of temperature. As a fluid never in equilibrium, 
it is in constant motion, and the consideration 
of its motions under various influences forms 
the main part of our study. Were the whole 
atmosphere of uniform density, equal to that 
which it has at the surface of the ground, its 
height would be only about 5 miles, and 
this height (20,000 feet) is sometimes called 



32 Weather Science 

the height of the " homogeneous atmosphere " ; 
this would produce a pressure equal to that 
actually existing at the surface of the earth, 
which pressure on every square inch is about 
equal to that produced by the weight of a 
column of mercury 30 inches high and 1 square 
inch in cross section. In terms of the C.G.S. 
system (in which the centimetre is the unit of 
length, the gramme the unit of mass, the second 
the unit of time) the air pressure on each square 
centimetre of the ground is about equal to that 
of a column of mercury 76 cm. high and 1 sq. cm. 
in cross section, upon an area of 1 sq. cm. at 
its base. 

This pressure is conveniently known as atmos- 
phere of pressure ; in English measure this equals 
about 147 Ibs. per square inch. The pressure 
higher up is less, and decreases gradually as 
we ascend. It is commonly and conveniently 
measured by the barometer (fidpvs, heavy ; perpov, 
a measure), perhaps the most important of all 
meteorological instruments. 

Though, as we have stated, the mean pressure 
is as above, yet its actual value is always vary- 
ing, and the study of these changes involves 
much of our science, sudden changes of pressure 
almost always accompanying changes of weather, 
whilst fairly settled or slowly changing conditions 
invarably indicate settled weather. 

Owing to the presence of the atmosphere, we 
do not see external (heavenly) bodies exactly in 



Refraction Effects 33 

their true positions. Rays of light coming from 
the sun, moon, or a star have their directions 
changed by this action, which is called " refrac- 
tion," just as we observe the image of a straight 
stick immersed partly in water to be apparently 
bent at the surface, the part below being seen 
raised above its true position. In a similar 
manner the sun and stars are seen above the 
position they would otherwise occupy in the 
sky, by a variable amount depending on their 
altitude above the horizon, and (in a less degree) 
on the temperature and other conditions of 
the air. 

There is an important difference, however, 
between refraction by the atmosphere and that 
produced by a fluid such as water. The latter 
being homogeneous, a ray of light entering it 
in any direction not perpendicular to its surface, 
is bent in a definite direction, making an angle 
with its original course (^ sine of angles of in- 
cidence and refraction in a constant ratio to one 
another"), and pursues its new course without 
further change so long as it remains in the 
water, whilst a ray of light entering our atmos- 
phere from outside undergoes a gradual and 
increasing deviation as it enters more and more 
dense air, whereby it is made to traverse a 
slightly curved path, and the celestial body is 
seen in the direction of the tangent to the 
point of the curve which reaches the eye of the 
observer. The density of the lower atmosphere 



34 Weather Science 

being greater than that of the upper air, the 
refraction is greater for the former. As a result 
of refraction the sun and moon are seen above 
the horizon at a time when they are really 
below it. The time of sunrise in our latitudes 
is accelerated, and that of sunset is retarded, 
and so the day is lengthened by from five to 
eight minutes. In more northern latitudes this 
difference is yet greater, whilst in the Arctic 
Regions the result is that the length of "per- 
petual day " is increased by several " days," and 
that of "perpetual night" is shortened by a 
corresponding amount. 

When the sun and moon are near the horizon, 
we notice that they are distorted from a circular 
form into ovals, and are usually of a reddish 
colour. The former effect is due to the fact 
that the amount of refraction changes rapidly 
near the horizon, being greater below than above. 
Thus the lower edges of these bodies are raised 
more than the upper ones, so that the vertical 
diameter is diminished. The horizontal diameter 
is not affected, and so the sun and moon appear 
to be oval and not circular. Their reddish colour 
is due to absorption. White light is composed 
of all the " colours of the rainbow " red, yellow, 
green, blue, violet, etc., and of these, the 
green, blue, and violet (the shorter "waves") 
are more readily absorbed by the atmosphere, 
or rather its vapours (for pure dry air exercises 
scarcely any absorption), than the red and 



Twilight 35 

yellowish rays (which are longer). Thus this 
process of selective absorption results in the 
loss of a greater amount of the blue and violet 
rays, and so the sun and moon, when seen low 
down near the horizon, appear reddish or 
yellowish rather than white, an effect seen also 
when the sun shines through a " November fog." 
Twilight is also a phenomenon due to the 
presence of our atmosphere. For a short time 
after the sun sets (and before it rises) some of 
its light falling in the upper regions clouds, 
etc., is reflected downwards, and thus gives 
illumination to the lower atmosphere and 
ground. It is not certain whether this reflect- 
ing power is due to the gases of the atmosphere 
or to the presence of dust particles, water 
vapour, and possible ice crystals existing in it. 
So long as the sun is not more than 18 below 
the horizon of any place, some light will thus 
be received by reflection from the upper atmos- 
phere, and thus the length of the day will be 
appreciably increased. It thus happens that 
from the latter part of May till the middle of 
July, though the sun is below our horizon at 
London for more than seven hours out of the 
twenty-four, there is no real night during that 
period, though at midnight the amount of 
light received from the sun is at times very 
small. The red tints of sunset are due to a 
similar cause to that from which arises the red 
colour of the rising and setting sun. 



36 Weather Science 

The generally diffused light of the sky in 
the daytime is also due to the presence of 
the atmosphere. If there were no atmosphere 
the stars would be visible by day as well as by 
night. The scattering and reflection of light by 
the air, its vapours and dust particles, give rise 
to a general illumination, strong enough to render 
the more feeble illumination of the stars invisible, 
except under special circumstances. The blue 
colour of the sky is also due to the scattering of 
light by small particles in the air, the red, as we 
have just seen, being transmitted ; though much 
of the other radiation forming white light is 
absorbed, a portion appears to be reflected. It is 
by no means certain what substances are most 
effective in this. Lord Rayleigh thinks that fine 
salt particles floating in the air, or even the 
oxygen itself may be the cause. The presence 
of metallic meteoric dust has also been suggested 
as an important factor in producing this colora- 
tion. Thus, apart from its essential character 
as a supporter of life and every kind of com- 
bustion, the other services rendered by our 
atmosphere are many and important. To its 
presence is due much of the varied play and 
coloration of the inorganic world, the brilliancy 
of the clouds, and the wondrous tints of sunrise 
and sunset ; the blue of the sky, so different from 
the inky blackness that would prevail were it 
absent, the gradual coming on and fading away of 
illumination in the morning and evening, instead 



Atmospheric Absorption 37 

of the instantaneous appearance of day and night 
respectively, are all results of its presence. 

As a storehouse of the solar heat after it has 
reached the earth, the atmosphere is a most 
important meteorological agent. The sun's rays 
pass fairly freely through the air, only a small 
portion being absorbed by the vapour of water 
and traces of carbon dioxide (oxygen and nitrogen, 
the main constituents being almost perfectly 
diathermanous or " transparent " to radiant heat), 
and reach the surface of the ground or the waters. 
Most of these rays are then reflected, but in the 
act of reflection they are changed somewhat in 
character, and are then largely absorbed by the 
water vapour and the clouds instead of going 
back into outer space again. Part of these 
absorbed and reflected rays are again reflected 
downwards by the clouds, and so in large measure 
the heat is kept in, and remains as a permanent 
gain to the earth. On a cloudy night the air 
never becomes so cold as on a clear one, when 
there is less vapour in the atmosphere. In the 
latter case the heat reflected from the ground 
not being stopped passes more or less completely 
out, and as none comes in from outside, the 
temperature rapidly falls. Theories of dew and 
hoar frost formation depend upon an action of 
this kind, as we shall see in a later chapter. 

Until quite recently all observations were 
confined to the denser layer of atmosphere in 
immediate proximity to the ground, and nothing 



38 Weather Science 

was known as to the conditions prevailing in the 
upper regions. At a few mountain observatories 
some information had been obtained, occasional 
balloon ascents were made, and inferences drawn 
from the behaviour of the upper clouds, which 
revealed the presence of currents whose velocity 
much exceeded anything known to occur near the 
surface of the ground. Of late years, however, 
by means of kites sent up with self-recording 
instruments, much important information as to 
these hitherto unknown regions has been obtained, 
some account of which will appear in a later 
chapter of this work. 

It has been already stated that there is no 
definite knowledge as to how far the atmos- 
phere extends upwards. The density, however, 
decreases very rapidly, and there can be very 
little air at a greater height than about 50 miles. 
Observations of the duration of twilight indicate 
about this limit, but meteorites visible only to us 
by their heating to incandescence by friction 
against a resisting medium, have been seen at 
a height of 100 to 150 miles, so that a very 
small amount of air must exist at a greater 
height than is shown by the twilight observa- 
tions. The aurora, a phenomenon probably due 
to electric discharges in the rarefied upper air, is 
not often known to exist at a greater height than 
about 40 to 50 miles. Though the extension of 
the atmosphere upwards is to a small degree un- 
certain, yet from a knowledge of its specific 



Weight of the Atmosphere 39 

gravity and pressure at the surface, its total 
weight may be inferred. 

One cubic foot of dry air, measured at the 
temperature of freezing water (32 F.) and under 
the barometric pressure equal to 30 inches of 
mercury, weighs 1*3 ozs. or 565 grains, more 
exactly, the exact experiments of Jolly giving 
for the weight of 1 litre of air at C. (82 F.), 
and under the pressure of 760 mm. (of mercury) 
values varying between 1*30493 and 1*30575 
grammes. The total weight of the atmosphere 
of the earth resting on its surface was given by 
Sir John Herschel as llf millions of pounds or 
5*3 trillions of kilogrammes (5'3x 10 21 grammes). 
The composition of this air is remarkably con- 
stant. Though a mechanical mixture, and not 
a chemical compound, the proportions of its 
ingredients vary very slightly, wherever the air 
may be collected. Of the four principal con- 
stituents, oxygen, nitrogen, argon, and water 
vapour, only the last varies to any appreciable 
degree. By volume, the oxygen forms 20*9 per 
cent., nitrogen 76*0 per cent., argon 1 per cent., 
whilst the water vapour varies from 1 per cent, to 
4 per cent., when the air is very damp. The 
carbon dioxide, whose amount in pure country 
air is less than 0*03 per cent., sometimes may 
exceed 0*07 or 0*08 per cent, in the air of a 
town, whilst in badly ventilated crowded halls 
it may reach to 0*20 per cent., or even more. 
Though these different gases differ considerably 



4O Weather Science 

in density, water vapour being the lightest, and 
carbon dioxide the densest ; owing to diffusion, 
the proportion of these ingredients does not 
sensibly vary at different altitudes. The heavy 
carbon dioxide is carried upwards, and the lighter 
water vapour downwards by the action of the 
same principle. Nevertheless, there is a tendency 
for the lighter gases to spread more rapidly up- 
wards, and the heavier to remain below, so that it 
has been supposed that the absence of hydrogen, 
the lightest of all gases, from our atmosphere in 
a free state, is due to the upward diffusion 
whereby it has " got beyond the power of recall 
by gravitation." But all such speculations de- 
pending as they do upon the uncertain indica- 
tions of the " kinetic theory " of gases, are to be 
deprecated, and we should rather wait for further 
experimental knowledge than attempt to supply 
its place by premature hypotheses. Within the 
last few years several new gases previously un- 
known have been discovered (argon, xenon, 
crypton, etc.), and hydrogen has been detected 
in the Jumerolles or jets of steam of Tuscany, 
and also in the human breath under certain 
conditions (Thorpe). 

The following quotation from the works of 
the late Dr Buist of Bombay may fitly be 
inserted in this place: 

" The atmosphere is a spherical shell which 
surrounds our planet to a depth which is unknown 
to us, by reason of its growing tenuity, as it is 



Buist on the Atmosphere 41 

released from the pressure of its own superin- 
cumbent mass. Its upper surface cannot be 
nearer to us than 50, and can scarcely be more 
remote than 500 miles. It surrounds us on all 
sides, yet we see it not ; it presses on us with 
a load of 15 Ibs. on every square inch of surface 
of our bodies, or from 70 to 100 tons on us in 
all, yet we do not so much as feel its weight. 
Softer than the softest down more impalpable 
than the finest gossamer it leaves the cobweb 
undisturbed, and scarcely stirs the lightest flower 
that feeds on the dew it supplies ; yet it bears 
the fleets of nations on its wings around the 
world, 1 and crushes the most refractory substances 
with its weight. When in motion its force is 
sufficient to level the most stately forests and 
stable buildings with the earth to raise the 
waters of the ocean into ridges like mountains, 
and dash the strongest ships to pieces like toys. 
It warms and cools by turns the earth and the 
living creatures that inhabit it. It draws up 
vapours from the sea and land, retains them 
dissolved in itself or suspended in cisterns of 
clouds, and throws them down again as rain or 
dew when they are required. It bends the rays 
of the sun from their path to give us the twilight 
of evening and of dawn ; it disperses and refracts 
their various tints to beautify the approach and 
the retreat of the orb of day. But for the 
atmosphere sunshine would burst on us and fail 
us at once, and at once remove us from 

1 This was written before the days when the atmosphere is 
polluted by the reckless waste of the stored up accumulation of 
ages ; the consumption of thousands of tons of coal, to satisfy the 
cravings of a few would-be ' ' record breakers/' to save a few hours 
of their worthless time, regardless of all other circumstances. 



42 Weather Science 

midnight darkness to the blaze of noon. We 
should have no twilight to soften and beautify 
the landscape, no clouds to shade us from the 
scorching heat, but the bald earth, as it revolved 
on its axis, would turn its tanned arid weakened 
front to the full and unmitigated rays of the 
lord of day. It affords the gas which vivifies 
and warms our frames, and receives into itself 
that which has been polluted by use, and is 
thrown off as noxious. It feeds the flame of 
life exactly as it does that of the fire it is in 
both cases consumed, and affords the food of 
consumption ; in both cases it becomes combined 
with charcoal, which requires it for combustion, 
and is removed by it when this is over." 

The temperature of the atmosphere is perhaps 
of more immediate interest to our physical well- 
being than almost any other phenomenon, for, 
as it has been well put by an esteemed friend 
of the writer, "the struggle for existence is 
essentially, after all said and done, a struggle 
with cold." Warmth is more than food to the 
body, and in fact the need for the latter is the 
want or otherwise of the former, food being 
the fuel of the living " steam-engine at work." 
Many eminent meteorologists, too, consider the 
determination of temperature as the most im- 
portant of all observations in the science, for it 
is to the unequal heating of different parts of 
the atmosphere, the sea, and the land, mainly 
by the sun's radiation, that almost all circulation 
and other movements are primarily due. From 



Tidal Action, Sunspots, etc. 43 

these arise the equatorial warm currents, both 
serial and aqueous, the polar colder streams 
which take their places, the differences of 
pressure in different regions (modified somewhat 
by the earth's rotation), the trade winds and the 
anti-trades, the various regions of high and low 
pressure (anticyclones and cyclones), etc. Of 
minor import, if indeed the action is certainly 
detectable, is such tidal effect as is produced by 
the unequal attraction of the moon and sun 
upon different parts of the atmosphere. Changes 
in the number and intensity of sun spots, and 
other signs of solar activity have relations more 
or less direct with the phenomena of terrestrial 
magnetism, the Aurora (Borealis and Australis), 
magnetic storms, etc., and have been suspected 
by some to have a yet more intimate connec- 
tion with weather conditions. Upon this point 
prejudice has hitherto perhaps come more into 
play than sober deductions from ascertained 
facts, one party rejecting as preposterous the 
notion that there can be any such connection 
between the greater or less development of sun 
spots and local weather conditions, the other 
party as unhesitatingly accepting it. Attempts 
have even been made to show that such things 
as the recurrence of famines, floods, excessive 
or deficient harvests, commercial "booms" and 
panics, are all intimately related to the sun- 
spot period. 

Such evidence as we have so far is of such a 



44 Weather Science 

nature that altogether contradictory results have 
been arrived at by different investigators, but 
some short account of these will be dealt with 
in a later chapter of this work (chap. xv.). In 
the "Dark Ages" the immediate influence of 
not only the sun and moon, but of all the 
heavenly bodies, upon terrestrial phenomena, was 
generally believed in. The phenomenon of dew 
was considered a product of the stars ; one planet 
" produced " rain, another gave fine weather, and 
so on. Even within comparatively recent times 
the " wild fancy " of Bishop Whiston attributed 
the Noachian deluge to the near approach of 
a "watery comet," which later on is to destroy 
the world by fire! This was offered to the 
world in Newton's day ! 

Till little more than two centuries ago comets 
and "shooting stars," still called meteors, were 
considered as terrestrial phenomena, and were 
accordingly not considered by astronomers, but 
as they are now known to be extra-terrestrial 
objects, they no longer fall within the scope of 
our science, and will not be further referred to 
here. A science of celestial meteorology, or 
the probable weather conditions existing in 
other worlds than ours, can scarcely be said 
to exist, though speculations have been often 
indulged in as to the habitability of the nearer 
planets. The phenomena of belts on the major 
planets Jupiter and Saturn have been likened 
to the trade -wind zones of our own earth, 



Objects of the Science 45 

etc. With all such matters, notwithstanding its 
etymology (TO, fjieTeupa, things above), meteorology 
does not deal, leaving to astronomy whatever 
may be known or imagined as to the physical 
and climatic conditions of the other bodies of 
the universe ; confining itself to the study of the 
physical state of our own atmosphere, its dis- 
tribution and temperature ; changes in these 
elements from day to day and throughout the 
year, their bearing on climate and habitability ; 
and, lastly, the possibility of inferring future 
changes from present conditions. 



CHAPTER II 

THE BAROMETER MERCURIAL BAROMETER GLYCERINE BAROMETER 
FORTIN'S BAROMETER ADJUSTMENTS SYPHON BAROMETER 

FISHERY BAROMETER ANEROID BAROMETER SELF-RECORDING 

BAROMETERS CORRECTIONS TO READING OF BAROMETER 

WEATHER VARIATIONS WITH BAROMETER OSCILLATIONS OF 

PRESSURE, DIURNAL AND ANNUAL EXTREME RANGE DOVfi's 

RULES REGIONS OF HIGH AND LOW PRESSURE PRESSURE AT 
VARIOUS ALTITUDES ABOVE THE SURFACE. 

THERE is probably no instrument to which the 
progress of modern meteorology is due so much 
as the barometer, the measurer of the weight, 
or rather pressure, of the atmosphere. The 
name is derived from the two Greek words, 
/3apv$, heavy, and M eVpoi/, a measure. It is an 
often-told tale that some Florentine workmen, 
finding that they were unable to raise the water 
in a pump to a greater height than 32 feet above 
its level in the well, came to Galileo to enquire 
the cause. The rise of water in a pump having 
been attributed to nature's abhorrence of a 
vacuum, he remarked upon this that it was 
evident that this abhorrence did not extend 
beyond 32 feet ! Torricelli, one of his favourite 
pupils, some time after Galileo's death, con- 
sidered that this water column was supported by 

46 



The First Barometer 47 

the pressure of the external air upon its surface, 
this forcing the fluid up to a height varying 
with the pressure and the density of the matter 
raised. A heavier liquid than water he thought 
would rise to a less height. Taking a glass tube 
closed at one end, and filling it with mercury, 
whose density is thirteen and a half times as 
great as water at the same temperature, he 
inverted this, holding it upright, over a larger 
vessel partly filled with mercury also. The 
liquid in the tube fell somewhat, part of it pass- 
ing into the vessel, and on measuring the height 
of the top of the liquid above the surface of the 
mercury in the vessel, he found this to be about 
30 inches. Thus was formed the first mercurial 
barometer. The space above the top of the 
mercurial column, between this and the top of 
the tube, was empty, and is known as the 
Torricellian vacuum. It is a nearly perfect 
vacuum, containing only a trace of mercury 
vapour. The height of the mercurial column 
was found to vary somewhat from time to time 
at the same place, and very considerably, when 
the apparatus was moved from place to place. 
Thus Pascal, the famous self-taught geometer, 
afterwards not less celebrated as a theologian, 
found that on the top of the Puy-de-D6me, one 
of the extinct volcanoes in the Auvergne region 
of France, the height of the mercurial column 
above the level of the cistern was only about 
25 inches as compared with 30 inches in the 



48 Weather Science 

town of Clermont. Thus it was shown that 
the smaller amount of air over the instrument 
on the mountain supported a shorter column 
than in the valley below. The earliest form of 
mercurial barometer differs but little from those 
at present in use ; the Accademia del Cimento 
employed a nearly closed cistern containing 
mercury into which dipped an upright glass 
tube graduated along its length and supported 
by the neck of the cistern. The most ordinary 
fluid for barometers is mercury, on account of 
its being much denser than any other known 
liquid, and giving off but little vapour at ordinary 
temperatures ; but other liquids have been also 
used. 

There is a glycerine barometer at the Geo- 
logical Museum in Jermyn Street, London, and 
water barometers have also been occasionally 
employed, but though the great length of these 
instruments enables minute variations in atmos- 
pheric pressure to be more easily seen, yet their 
very length is itself a disadvantage, and the 
amount of vapour given off thereby producing 
a depression of the barometric column vary- 
ing with temperature and other conditions, 
necessitates the use of troublesome corrections 
before the true pressure can be obtained. In 
addition, there is the very considerable effect 
of capillary "attraction." On the other hand, 
mercury is a liquid which may be easily obtained 
reasonably pure, and its rate of expansion for 



A Simple Barometer 49 

different temperatures is well known and very 
regular within the ordinary range employed. 
Of the various forms in use we propose to 
describe only one or two of the most ordinary. 
The simplest kind of barometer, as already 
mentioned, consists of a glass tube supported in 
an upright position over a cistern nearly full of 
mercury, its lower end dipping into the latter. 
The barometer tube is filled nearly full with 
mercury, and is then heated till the fluid boils. 
Thus any air or moisture contained in the tube 
is expelled. It is thus completely filled with 
mercury, and inverted in the cistern. The 
atmospheric pressure at any moment is measured 
by the height of the top of the mercury column 
above the surface of the liquid in the cistern. 
A scale whose zero point is at the level of the 
latter surface, divided into inches and fractions 
of an inch (or into millimetres), affords the 
means of doing this. As, however, the mercury 
in the tube is supported by the pressure of the 
atmosphere, and the latter varies continuously, 
the length of the column must also vary accord- 
ingly ; consequently the level of the mercury in 
the cistern must also change, for when the fluid 
rises in the tube its level must fall in the cistern, 
and vice versa ; thus the position of the surface 
in the latter must change, falling when the height 
in the tube increases, and rising when mercury 
flows from the tube into the cistern. Thus, to 
allow for this change of level, an arrangement, 



Weather Science 



such as that adopted for the well-known Fortin 
form, must be adopted. This 
instrument is a cistern barometer, 
containing mercury as the fluid, 
with a scale whose lower end is 
a fixed ivory pencil. Before read- 
ing the instrument, in most cases 
the point will either be below the 
surface of the mercury in the 
cistern or else slightly above, 
but by means of a screw at the 
bottom of the cistern the latter 
may be raised or lowered till the 
point of the ivory pencil is in 
contact with the mercury surface, 
which is shown by the apparent 
contact of the point and its image 
reflected from the liquid metal. 

The reading of the scale at 
the top of the upper surface will 
then give the apparent barometric 
height. Another adjustment is 
necessary, however, before this 
reading can be ascertained. It 
will be noticed that the surface 
of the top of the mercury column 
in the tube is not flat but curved, 
slightly convex upwards, being 

standard Barometer, highest in the centre. The read- 

(Fortin's pattern.) ing of the scale ^ ^ highest 

point must be taken, the eye must be as nearly 



The Scale and Vernier 51 

as possible on the same level, and most instru- 
ments are provided with a vernier, whereby 
the reading may be made with a greater degree 
of accuracy. In barometers in ordinary use 
in this country the scale is divided into inches, 
lOths and 20ths of an inch. The vernier is a 
small movable scale divided into a number of 
equal divisions, such that a fixed number of 
these are equal in length to one more or one 
less than a number of divisions on the fixed 
scale. Very frequently, as we have stated, 
the scale of the barometer is divided into 20ths 
of an inch, and twenty-four of these divisions 
correspond to twenty-five spaces on the vernier, 
whereby $ * $ = -gfo inch may be read off, 
or y^^j inch by estimation. The bottom of 
the vernier being brought exactly on a level 
with the convex top of the mercury column, the 
reading is made. The line on the scale next 
below this point is noted, and this gives the 
nearest ^ inch ('05), the smaller fractions being 
taken from the vernier. Thus suppose that the 
line on the main scale next below the bottom of 
the vernier is 29 '550 inches, and that the next 
line above the 3 on the vernier coincides with 
one on the main scale, when the instrument is 
set each long line on the scale corresponds to 
1 inch, and the intermediate short lines '05 inch, 
every long numbered line on the vernier gives 
the -01 (lOOths), and the short lines '002 (2,000ths 
of an inch.) Then the reading is 29 -550 + -03 + 



52 Weather Science 

002 = 29,582 inches. The attached thermometer 
is to give the temperature at the time of the 
reading, for in reducing the reading of the 
barometer the correction for temperature is a 
very important one. This thermometer should be 
read before the barometer. Of other forms the 
best known are the "syphon" barometers, the 
" fishery " barometer, and " aneroid " barometers. 

The syphon barometer, as its name indicates, 
has a tube bent up into a U-shaped form, a 
short open end about 6 to 8 inches long, and 
a longer closed end, the whole supported in a 
vertical position, the two ends upwards. There 
is no need, therefore, for a cistern in this form of 
the instrument, for when mercury is introduced 
into the open end it runs down and gradually 
rises in the other limb, the atmospheric pressure 
on the open end supporting a column in the 
closed part, whose height varies according to 
the variations in the former. 

The distance between the levels of the mercury 
in the open and closed ends respectively gives 
the height of the barometer, and scales are pro- 
vided to enable the necessary measurements to 
be made. If properly made with a correct scale 
and uniform tube, no correction for capillarity 
will be required for this instrument, the only 
corrections being those for temperature and 
altitude above (or depression below) mean sea- 
level. This form of instrument, however, is not 
so much in use in this country as on the 



Fishery and Aneroid Barometers 53 

Continent. A thermometer is mounted to the 
same framework to give the temperature of the 
instrument at the time of reading. 

The fishery barometer was designed by the 
late Admiral Fitzroy, some time Director of 
the Meteorological Office. It is a mercurial 
barometer fastened to a box-wood frame, the 
mercury column dipping into a box-wood cistern 
which has a flexible sheep - skin base and is 
provided with a lifting screw. There are two 
verniers, reading to hundredths of an inch, one 
being placed on each side of the tube, and a 
large attached thermometer. Many of these 
instruments are erected at exposed positions on 
the coasts, at coastguard stations for the " use 
of fishermen, sea-faring persons, and the public 
generally" (whence the name). The lettering 
on the instrument is the result of considerable 
experience, and is more accurate and reliable 
than that on the ordinary "weather glass." 
These instructions, from the name of their 
designer, are known as Admiral Fitzroy's rules. 

A common and portable form of barometer 
is the well-known aneroid, usually made in the 
form of a watch or chronometer. In this instru- 
ment the pressure of the atmosphere is measured 
by means of its effect in altering the shape of 
a metallic box from whose interior the air has 
been partly removed, the upper surface of the 
box being corrugated to make it yield more 
easily to external pressure. At the centre of 



54 



Weather Science 



the top of the box there is a pillar connected 
with a powerful spring, to keep the box from 
collapsing. The top of the box rises or falls 
with the variations of pressure, and these move- 
ments are transferred by means of levers and 




Aneroid. 

springs to a hand which moves on a dial like a 
clock face. The instrument has to be graduated 
by comparison with a mercurial barometer ; it is 
very quick in indicating changes of pressure, and 
may be made small enough to go in the waist- 
coat pocket, and is thus very convenient for 
mountain observations, etc. 

It has the drawback of being affected by 



Wheel Barometer 55 

changes of temperature to an uncertain degree, 
each instrument acting differently in this respect, 
and, moreover, it is liable to gradual changes 
which necessitate its frequent comparison with a 
standard mercurial barometer, so that for exact 
scientific work there are serious objections to 
its use. 

The wheel barometer consists of a mercurial 
syphon barometer, whose two branches have 
usually the same diameter. On the surface of 
the mercury in the open branch there floats a 
piece of metal or glass suspended by a thread, 
to the other end of which is fixed a pulley on 
which the thread is partly rolled. Another 
thread, rolled parallel to the first, supports a 
weight which balances the float. A needle 
moving on a dial is fixed on the axis of the 
pulley, and the float moving with the rise or 
fall of the mercury, the pulley turns and the 
needle with it, thus recording the variations of 
pressure. The instrument has the disadvantage 
resulting from the friction arising from this 
additional apparatus, and is consequently slow 
in indicating changes. 

Self - recording mercurial and aneroid baro- 
meters or " barographs " are in use at large fixed 
public observatories, and some of the cheaper 
forms are used also by private observers. 

They usually need comparison with standard 
mercurial barometers, and when this is regularly 
done their records, as giving the continuous 



56 Weather Science 

variations of pressure for every moment, are of 
great value in detecting minute irregularities and 
variations of short period, which would otherwise 
escape notice. The photographic method in use 
at Kew and Greenwich consists in recording 
photographically the varying position of the 
top of a mercurial barometer. Sensitized paper 




Barograph. 

is wrapped round a cylinder, which is turned 
with uniform motion. Usually one complete 
revolution of the cylinder is performed in a 
day. Light from a lamp behind the barometer 
falls upon the paper, but is intercepted by the 
mercurial column to a greater or less degree, 
the line of division between the part acted upon 
by the light and the unaffected part varies 
according to the varying height of the column, 
every part of the cylinder, except that on which 



Corrections to the Barometer Readings 57 

the spot of light falls, being covered with a case 
of blackened metal. Thus there is traced upon 
the paper a boundary curve whose ordinates 
(heights) are proportional to the movement of 
the barometer, and on developing the image the 
trace becomes visible to the eye. 

The readings of every barometer, to bring the 
indications of different instruments into harmony, 
need various corrections before they can be 
applied for comparison, or other purposes. They 
are usually given as five (or six) in number. 

I. Correction for Index Error. 
(II. Capacity). 

III. Capillarity. 

IV. Temperature. 

V. Altitude above Sea-level. 

VI. to reduce to mean latitude 45. 

Of these the first three corrections vary for 
different instruments, and are often previously 
obtained by comparison with standard instru- 
ments (II., does not apply to Fortin's baro- 
meter), whilst the corrections IV. and V. are 
conveniently applied by means of tables, since 
they are the same for all instruments under 
similar conditions. 

The Correction for Index Error is the amount 
to be applied to the readings owing to the fact 
that the scale (inches or millimetres, etc.) is not 
quite accurate, so that a given reading, say 29 
inches on the standard barometer, corresponds 



58 Weather Science 

to a somewhat lower or higher reading on the 
instrument under comparison. 

It is stated that some instruments have been 
found to read half an inch too high, others as 
much too low (Scott). Others again, which are 
correct in one part of the scale, are found to be 
several tenths of an inch wrong in other parts. 
The usual practice at Kew is to test them at 
every half inch from 27*5 to 31 inches (of 
mercury), and the corrections thus found are 
registered in a table to be used with that 
particular instrument. Any barometer whose 
index error is greater than 0-010 inch is to be 
rejected (Kew, 1875). 

The Correction for Capacity. In barometers 
with closed cistern there is a certain height for 
the mercury at which the column is correctly 
measured by the scale. Below this, since when 
the mercury sinks in the tube it rises in the 
cistern, the height read off must be too great ; 
and, conversely, the level in the cistern falling 
as the mercury rises in the tube, readings will 
be too low when this is the case. When the 
ratio between the capacity of the tube and the 
cistern is known we may allow for this as follows : 
Suppose the capacity of the tube to that of the 
cistern to be T foj, and we know the height at 
which the reading of the scale is correct, this is 
called the " neutral point." Then by adding a 
T ^jth part of the difference between the height 
read off and that of the neutral point, we get the 



Capillarity and Temperature 59 

true reading when the column is higher whilst we 
subtract the difference when the column is lower. 
It will be remembered that Fortin's barometer 
having a movable cistern, the zero of the scale 
is always at the level of its surface, and so no 
such correction is wanted for instruments of that 
pattern. 

The Correction for Capillarity arises from the 
fact that there is what is called a capillary 
repulsion between mercury and glass. The 
surface of the former in a glass vessel is never 
horizontal, but is lower at the edges than at the 
centre, and, moreover, this depression is greater 
in narrow tubes than in broad ones, being nearly 
inversely proportional to the diameter of these 
latter. It has also been found to be greater in 
tubes in which the mercury has not been boiled 
than in those where it has been so treated. 

But the more important of the corrections 
to be applied are those for temperature and 
altitude, to be next described. 

The Correction jor Temperature, or Reduction 
to Freezing Point (32 F. or C.). The attached 
thermometer gives the temperature at the time 
of observation. Since all bodies are affected by 
change of temperature, usually expanding with 
its increase and contracting with decrease, the 
length both of the mercury column and of 
the scale itself will vary with variations in 
temperature, apart from the rise or fall of the 
mercury due to pressure changes. Mercury 



60 Weather Science 

being a liquid, is more expansible by heat than 
either glass or the scale, and consequently the 
level of the barometer will rise with any rise in 
temperature and fall with a fall of temperature, 
even though the air pressure remain constant. 
All readings of the instrument are accordingly 
reduced to the temperature of the freezing 
point (32 F.), and tables are given which enable 
the correction to be applied to the observed 
length at other temperatures to reduce them to 
the value they would have at that standard 
temperature, the temperature of the instrument 
at the time of observation being given, of course, 
by the attached thermometer. 

Correction jor Reduction to Sea-level. The 
height of the barometric column, being a measure 
of the pressure of the atmosphere, is greater or 
less, according as the latter varies, and so it is 
evident that if we ascend above the level of 
the surface of the ground, there being less air 
as we go up, the pressure will be less, and 
consequently the barometer will fall, whilst, 
conversely, if we go down into a mine, the air 
pressure will be greater than that at the surface. 
For moderate elevations above the surface the 
barometer falls about ^ of an inch for each 
100 feet of altitude, but this amount varies not 
only in different places, but also with variations 
of temperature. We may determine the height 
of a mountain or other elevated station by 
readings taken at the sea-level and the station 



Reduction to Mean Latitude 61 

respectively, and the barometric determination 
of heights depends upon this comparison of 
readings. Tables of more or less exactness are 
given to enable the correction to sea-level to 
be made, and for the British Isles the mean 
sea -level at Liverpool (Ordnance Survey) or 
"high water at London Bridge," is taken as 
the zero of altitude. 

The last "correction," Reduction to Mean 
Gravity at Latitude 45. To reduce the read- 
ings of the barometer to a standard of com- 
parison available all over the globe, it has been 
proposed to take as the standard the value of g, 
the acceleration due to gravity, at latitude 45, its 
value varying slightly at different latitudes from 
32-086 in foot second units (or 977*99 C.G.S. 
units at the Equator to 983*21 at the Pole) at 
the Equator to 32*258 at the Pole, and multiply 
the barometric readings by the proper factor, 
ratio of gravity at the station to that at latitude 
45. For latitude 50 this is nearly equivalent 
to a correction (addition) of +*014 inches to the 
barometer at 30 inches, and about +'013 inches 
(barometer 29 inches). This correction, however, 
is not often applied, and, of course, is only for 
mercurial barometers and not for aneroids. In 
practice the corrections for index error, capacity 
(if required), and capillarity are contained in one, 
the "Kew correction," when the instrument has 
been tested at that institution, and after this 
is made, the corrections for temperature and 

D 



62 Weather Science 

altitude are next applied. As an example of 
the way in which these corrections are made, 
we may take the following, given by Mr 
Scott : 

f Attached thermometer 68 F. 

Uncorrected reading . . 29 '946 in. i Altitude of cistern, 105 feet 

^ above sea-level. 

Kew correction . . . -HH* ' 



29-960 

(Deduct) Temperature cor- 1 .-,/. 
rectionfor68 \ " 106 

29-854 = (Reading at 32 F.). 
Altitude of 105 feet at tern- ") 
perature 50 and approxi- > + -116 
mate pressure 30 inches J - (Readi correc ted and re- 

29 ' 970 i duced to 32' F. and M. S. L. ). 



Note. In applying the correction for altitude we must know the 
air temperature at the time, for the difference of pressure between 
the sea-level and the place of observation is for the height of the 
vertical column between the two positions, and the weight of this 
column varies with its temperature, being less for hotter air than 
for colder. 

The old and familiar name of " weather glass " 
for this instrument has already been alluded to 
as well as the notation engraved on the scales 
of many barometers. 

This usually appears in accordance with the 
idea of high barometer for fine weather and low 
barometer for bad, something as follows : 



31 inches = very dry. 
30-5 = set fair. 
30 = fair. 



29 inches = rain. 

28 ! 5 =much rain, 

28 = stormy. 



29 '5 a= change. 

Apart from the fact that these terms are of 



Diurnal Variations of Barometer 63 

very limited application, being only roughly true 
for places at mean sea-level and under normal 
conditions, it often happens that rain falls with 
a high and rising barometer, and we frequently 
get fine weather when the latter is low, so that 
this notation is practically worthless. The fishery 
or Fitzroy barometers, instead of the above mis- 
leading and imperfect notation, have instructions 
engraved on their scales applicable to the usual 
course of the weather in these islands. 

In a general way the barometer in these 
latitudes varies most irregularly, but in tropical 
regions, apart from occasional storms, there is 
a regular daily rise and fall. It generally rises 
from a minimum at 4 A.M. to 10 A.M., when it 
attains its first maximum, then falling again to 
4 P.M. second minimum, rising again to a second 
maximum at 10 P.M. So regular is this oscilla- 
tion that it is sometimes stated that the hour of 
day can be ascertained by the height of the baro- 
meter. Any irregularity in this daily oscilla- 
tion is an unfailing sign of a storm. The 
morning maximum and the afternoon minimum 
(10 A.M. and 4 P.M.) differ more from the mean 
than the other oscillations. At Calcutta the 
diurnal range, or difference between the extreme 
values, is about 0*12 inch. This range is very 
much smaller at stations in higher latitudes, 
but has been detected everywhere where the 
alternation of day and night exists (Herschel), 
and is evidently due (partly) to the action of 



64 Weather Science 

the sun, "though the causes are not yet 
thoroughly worked out." There is a prob- 
ability, however, that the moon, too, has an 
action here, and that, like the ocean tides, both 
sun and moon are instrumental in producing 
an "atmospheric tide." On this point letters 
by Mr Dines and Mr Langdon in Symons' 
Meteorological Magazine, April 1910, may be 
consulted, under the heading of "Atmospheric 
Tides." 

"It is fairly certain that the natural period of 
oscillation of the atmosphere as a whole is 
about twelve hours. Thus the tide-producing 
power of the sun must inevitably produce some 
such tide as we see in the double daily baro- 
metric oscillation." DINES. 

It appears, from examination of various series 
of long - continued observations, that there is 
also an annual period as well as a daily one, but 
this variation is small for our islands, though 
for continental regions it is sufficiently well 
marked. The characteristic high barometer 
over central Siberia during the winter, as com- 
pared with the much lower readings during the 
summer in that country, are well known. The 
same thing is generally true to a less degree of 
the mean pressure over North America during 
the same seasons. 

The extreme range at sea-level in our latitudes 
may be roughly taken as about 3 inches, from 
28*0 to 31*0 inches of mercury ; its mean height 



High and Low Readings 65 

being rather under 30 inches, " one atmosphere " 
being conveniently expressed as the pressure 
equal to this value. More exactly, the value of 
the standard pressure is defined as such that 
it will support a column of mercury 76 cm. 
high, at latitude 45 and at the sea-level, the 
temperature of the mercury being C. (32 F.) 
( Watson's " Physics "). For our latitude (51 N.) 
the standard pressure usually adopted is 29*905 
inches of mercury, under which pressure the 
boiling-point of water is 212 on the Fahrenheit 
scale, which is slightly smaller than the value 
76 cm. for the latitude of Paris, corresponding 
to 29-922 inches. 

It is not often that readings of the barometer 
under 28 -5 inches or over 30 -5 are recorded at 
stations on the sea-level. Occasionally, during 
storms, readings under 28 inches are taken, as at 
Ochtertyre in Scotland, during the great storm 
of 26th January 1884, when a reading of 27*332 
inches was recorded, and throughout the day 
readings under 28 inches were taken in the north 
of Ireland and Scotland. As examples of high 
barometric readings, I have myself on several 
occasions observed the barometer standing 
slightly over 31 '0 inches at Markree, at the end 
of January 1902. Much higher readings have 
been observed during the long continuance of 
the winter anticyclone over Siberia, 31'62 inches 
having been recorded at Barnaoul in December 
1877. 



66 Weather Science 

Professor Dove's practical rules on the relation 
of barometric changes and wind direction may 
be thus condensed (for Europe) : 

The barometer falls with east, south-east and 
south-west winds. It rises with west, north-west 
and north winds. It ceases to fall with a south- 
west wind and begins to rise ; with a north-east 
wind it ceases to rise and begins to fall. These 
rules were originally given for the whole Northern 
Hemisphere, but it has been shown that for Asia 
and the extreme northern regions of America 
and Greenland, they are not correct (Scott). 
Similar rules for the Southern Hemisphere by 
interchanging of north and south, were pro- 
pounded by the same authority. 

Dr Schreiber of Leipzig has, however, shown 
that these relations are not necessary, that wind 
direction and barometric pressure are probably 
more complex functions of the general weather 
conditions. The warmest currents are usually 
the dampest, and the relative humidity and 
probability of rain are roughly inversely as 
barometric pressure, but have also a minor rela- 
tion to wind direction. Thus we have the usual 
relation between a higher rising barometer and 
fine weather and falling barometer with rain 
and wind ; the fall being greater for high winds 
than for heavy rains. Most generally a rise 
of the barometer is accompanied by a fall of the 
thermometer and vice versa, but whenever a 
simultaneous rise of both instruments occurs it 



"High" or "Low" Barometer 67 

may be taken as "a sure sign of steady fine 
weather." The contrast between the weather 
conditions implied by the slow or rapid move- 
ments of the barometer is well expressed in the 
following couplet: 

" Long foretold, long last ; 
Short notice, soon past." 

A rapid rise or alternate rising and falling 
indicates unsettled weather conditions, whilst a 
gradual rise foretells fine weather, and a gradual 
fall during fine weather is a promise of a spell 
of continued bad weather. With regard to the 
amount of rise or fall of the barometer in a 
given time it may safely be said that a fall of 
^ inch in an hour, or f 3 ^ inch within three or 
four hours, is a sure indication of a coming storm 
(Chambers). 

Professor Mohn in his " Meteorologie " sum- 
marises the general reasons for a high or low baro- 
meter (apart from storm and violent changes) as 
follows. The barometer is high when the air is 
very cold, the lower strata being more dense 
than when it is warm, and more contracted, thus 
causing the upper strata to sink down and bring 
a greater mass of air, thus increasing the pressure 
at the base (recorded by the barometer). When 
the air is dry it is denser than if partly composed 
of moisture (the vapour of water having only 
62 the density of air at the same temperature 
and pressure), and thus the barometer is high. 



68 Weather Science 

Whenever an upper current sets in towards a 
given area, compressing the lower strata, this 
also causes a high barometric pressure. On the 
other hand, the barometer is low when the lower 
strata are heated, the isobaric surfaces (surfaces 
of equal pressure) are elevated, and a barometric 
slope causing motion results, so that the mass of 
air pressing on the lower regions is reduced. 
When the air is damp its density is reduced, 
and the more so the greater the amount of this 
moisture, since, as we have already stated, water 
vapour at the same temperature and pressure 
has only f ths the density of dry air. Whenever 
the air has an upward tendency, its pressure is 
lessened, so the barometer reads lower. 

The presence of deep snow is another favour- 
able condition for a high barometer, for this tends 
to prevent changes (rises) of temperature by in- 
terposing a non - conducting layer between the 
air and the ground. 

We accordingly find that the regions of highest 
barometrical readings are situated in the interior 
of continents and in high latitudes at the time 
when the temperature is lowest, as, for instance, 
in Eastern Siberia during the winter, where a 
constant anticyclone prevails for many months. 
The much greater proportion of water to land in 
the Southern than in the Northern Hemisphere, 
whilst it tends on the whole to raise slightly the 
mean temperature of places in southern latitudes 
over those of corresponding regions north of the 



Difference of Hemispheres 69 

Equator, similarly tends to lower the pressure 
over the former. Between latitude 40 S. and 
the South Pole there is little land beyond a 
few scattered islands, with the exception of 
the supposed Antarctic continent, which in any 
case is probably much smaller than the land 
round the North Pole. Thus the atmosphere 
over a large portion of the Southern Hemisphere 
is more heated than at equal latitudes of the 
northern half of the globe, so that the " isobaric 
surfaces " are raised and the differences of pressure 
between the Equator and higher latitudes 
diminished. This difference of pressure being 
the force tending to produce a flow from the 
Equator towards the polar regions, a smaller 
mass will ascend, and consequently the pressure 
(due to the weight of the superincumbent atmos- 
phere) will be correspondingly less. The presence 
of any considerable mass of land near the South 
Pole must act as a " centre of indraught " for 
the upper currents from the warmer water 
surfaces around it, thus tending to still further 
reduce the pressure. To this defect of pressure 
is also attributed the exceptional force and 
frequency of the " brave westerly winds " which 
blow so persistently over the Southern Ocean. 

[Note. The variations of barometric pressure at different heights 
above the sea-level have been determined, and a table prepared by 
Dr Sprung gives the annual mean pressures, their mean values for 
January and July, and also for altitudes of 2,000 and 4,000 metres 
(6,700 feet and 13,000 feet), above the surface for 5 intensity of 
latitude from 80 N. to 70 S. From this table we find a minimum 
annual mean of 758 -2 mm. at latitude 65 N., increasing thence to 



yo Weather Science 

a maximum 762*4 mm. at 35 N., again decreasing to 757*9 mm. 
at latitude 10 N. From this to southern latitudes the mean annual 
pressure increases to 763 '5 mm. at latitude 30 S., falling off again at 
higher latitudes (738-0 at latitude 70 S., the most extreme southern 
latitude given). At an altitude of 2,000 metres we have for latitude 
80 N. 582 '0 mm., increasing at first somewhat more rapidly to 
600-9 mm. (latitude 30 N.), then more slowly, till in latitude 20S. the 
maximum 602'7 is reached, after which there is a fall, at first slow, 
then more rapid, to 569*9 in latitude 70 S. The pressures at altitude 
4,000 metres increase from 445*2 (latitude 80 N.) to 471*1 (latitude 
10 to 20 S.), and then decrease to 437 '2 at latitude 70 S. Thus 
for the Northern Hemisphere from this table we get a maximum 
pressure at latitude 30 N., a slight minimum at the Equator, and 
a more pronounced minimum at latitude 65 N., with a slight 
increase northwards, onwards towards the Pole. At elevations 
2,000 metres the maximum pressure is nearer the Equator, and 
the same is the case with the observations for 4,000 metres altitude, 
there being only one minimum for each towards the polar regions, 
no increase of pressure in the highest latitudes being evidently such 
as the observations on the surface indicate. Sprung's numbers are 
given on the metric units ; they may be reduced to British statute 
feet, and inches of pressure by remembering that 39 -37 inches = 1 
metre.] 



Invention of the Thermometer 71 



CHAPTER III 

THE THERMOMETER '. SCALES, FAHRENHEIT, CENTIGRADE, REAUMUR, 
DE I/ISLE THE HYPSOMETER Cf WET AND DRY BULB " HYGRO- 
METERS DE SAUSSURE'S HAIR HYGROMETER MAXIMUM AND 

MINIMUM THERMOMETERS SELF-RECORDING THERMOMETERS 

(THERMOGRAPHS) THE STEVENSON SCREEN GLAISHER SCREEN 
BLACK BULB THERMOMETERS RANGES IN TEMPERATURE. 

THE thermometer seems to have first made its 
appearance about the end of the sixteenth 
century, and its invention has often been 
attributed to Galileo, though there is reason 
to believe that some kind of instrument for 
measuring temperatures was in use before his 
time. Galileo, however, used at Padua a glass 
bulb with a narrow open tube attached to it. 
This bulb was heated, and the open end of 
the tube placed under water in a vessel. The 
water rose to a certain extent in the tube, 
and as the air in the bulb cooled it rose still 
further, falling again when the bulb was warmed, 
exactly as the modern differential thermometer. 
About 1630 a French physician suggested 
inverting the Galilean thermometer, nearly fill- 
ing it with a liquid, and observing the rise and 
fall of the liquid with changes of temperature. 



72 Weather Science 

The "Florentine thermometer," invented by 
pupils of Galileo, consisted of a bulb with a 
fine tube above, and the fluid employed was 
alcohol ; the tube divided arbitrarily by means 
of small glass particles attached to its side, 
the zero being at about the point to which the 
liquid fell in a freezing mixture of salt and 
water. The substitution of mercury for alcohol, 
first suggested by Halley, has such obvious 
advantages that the former is now always 
used for exact observations, except for such 
temperatures as are occasionally met with in 
very cold countries, where mercury freezes in 
the winter. Mercury is a substance which can 
be obtained with considerable purity, and is 
liquid throughout the ordinary range of meteoro- 
logical observations. In addition it has a low 
specific heat and great conductivity, so that it 
is quickly affected by changes of temperature, 
and soon indicates these. The first use of the 
freezing- and boiling-points of water as fixed 
temperatures is variously attributed to Newton 
and Fahrenheit, the latter of whom also divided 
the interval between these temperatures into 180 
parts or degrees. Finding that the temperature 
of a mixture of snow and salt was 32 of these 
degrees below the freezing-point of water, he 
called this latter the zero temperature (perhaps 
supposing no lower temperature to be possible), 
and the freezing-point being thus 32, the 
boiling-point became 212. This scale is in 



Scales in General Use 73 

almost universal use in this country, though 
originally "made in Germany," and has the 
advantage that in expressing observed tempera- 
tures it is very rarely necessary to use the 
negative sign (-) for temperatures below zero, 
whilst the small size of its degrees is also 
useful. 

The scale, however, which is in general use 
throughout Europe is that known as the 
Centigrade. In this, the interval between the 
freezing- and boiling-points of water is divided 
into 100 degrees, the freezing-point being (or 
zero) and the boiling-point 100. It is commonly 
stated that the invention of this scale was due 
to Celsius, but it appears that the credit should 
be given to Linnaeus (Scott, " Meteorology," p. 
19). The former called the boiling-point 0, and 
the freezing-point 100, the degrees running 
downwards, whilst the suggestion of Linnaeus 
that the freezing-point should be called 0, and 
the boiling-point 100 agrees with the present 
universal practice. A third scale, proposed by 
Reaumur, is still sometimes used in Germany 
and Russia. On this the freezing-point is 0, 
but the boiling-point is 80, the degrees being 
thus larger than on the centigrade scale. A 
fourth system, proposed by Dr De L'Isle in 
1733, divides the interval between the freezing- 
and boiling-points of water into 150 degrees 
(but this is nowhere in use at present so far as 
we are aware), reckoning backwards from the 



74 Weather Science 

boiling - point. Rules are sometimes given 
whereby temperatures on one scale may be con- 
verted into corresponding readings on any or all of 
the others, but being a matter of pure arithmetic 
and of very little general utility, we need not 
waste space to do that which any reader can 
do for himself. It is to be hoped that in a 
few years' time all save one scale will be mere 
matters of antiquarian interest. At present, 
however, it is necessary for the English reader 
to acquaint himself with the Fahrenheit and 
Centigrade scales, the former because it is in 
general use in this country, the latter because 
almost all continental writers use it, and it is 
even gaining ground here also, being generally 
adopted by chemists and physicists, if not by 
meteorologists. 

Under ordinary conditions we may look upon 
the freezing-point as a constant temperature, 
since it is only slightly affected by variations 
in atmospheric pressure, being only lowered 
0-0075 C. = 0-0135 F. by one "atmosphere" 
increase of pressure, and for the ordinary varia- 
tions the depression or elevation is smaller 
than the probable error of the thermometer. 
On the other hand, the boiling-point of water 
is greatly affected by variations in the intensity 
of the superincumbent atmospheric pressure, 
and a difference of 1 inch above or below the 
standard (usually taken at 30 inches, or more 
exactly 29*905 inches of mercury) raises or 



The Hypsometer The Hygrometer 75 

lowers the boiling-point by 17 F. (on the 
metric system an increase of 27 mm. in pressure 
corresponds to 1 C. rise in boiling-point, and 
conversely). On this variation in the boiling- 
point depends the hypsometer, an instrument for 
the measurement of height (Greek ^09, height ; 
/merpov, a measure) : which is nothing but a 
boiling-point thermometer with a large scale 
whereby the temperature at which water boils 
on a mountain may be easily ascertained by 
performing the experiment. Then from the 
tables the barometric pressure corresponding 
to the ascertained boiling-point is known, and, 
with less exactness, the height of the mountain 
may be inferred. As, however, we know that 
the barometric pressure at any point on the 
surface varies within a range of 2 or 3 inches, 
the results of hypsometrical observation are 
liable to considerable uncertainty. 

The simplest form of hygrometer, or instru- 
ment for measuring the amount of vapour 
present in the atmosphere at any given time, 
consists of the " wet and dry bulb " arrange- 
ment. This is merely two similar thermometers 
mounted side by side on a frame (often inside 
a Stevenson's or other screen), the bulb of 
one being wrapped in muslin or other soft 
material, whilst the other thermometer has 
its bulb uncovered. The muslin is tied round 
the bulb by means of a few threads of worsted 
or cotton, the other ends of which dip into 



76 Weather Science 

a small vessel of water, which is kept fairly 
full. The two thermometers being compared, 
it will be found that the " wet bulb " instrument 
will in general read somewhat lower than the 

other. So long as the air 
is not saturated with 
moisture, some of the 
water carried up by the 
threads to the muslin 
will be continually evap- 
orating, and this evapora- 
tion causing an absorp- 
tion of " latent heat," the 
temperature of the instru- 
ment will be loVered 
thereby. When the air 
is saturated at any given 
temperature no more 
moisture can evaporate, 
and so the wet bulb and 
dry bulb thermometers 
will give the same read- 
ing. Thus we may see 
that the difference be- 

Wet and Dry Bulb Hygrometer. 




a measure of the amount of vapour present 
in the air at any given time. The theory of 
this instrument, sometimes called the " psychro- 
meter " (V^X/ 00 '^ cold ; perpov, measure), has been 
investigated by Apjohn, August, and others, and 
tables have been formed by Glaisher, as the 



The "Hair Hygrometer" 77 

result of experiments carried on at Greenwich 
and elsewhere, whereby from the observed read- 
ings of the dry and wet bulb thermometers 
the amount and pressure of the aqueous 
vapour of the atmosphere may be obtained, 
approximately at least for our latitudes. 

Regnault's and De Saussure's hygrometers 
are but little used in this country, though the 
latter instrument was recommended by the 
Vienna Congress of Meteorologists. This, the 
" Hair Hygrometer," consists essentially of a 
human hair, which is fixed at one end and 
stretched by a small weight at the other, the 
cord of connection passing over a block to 
which is attached a pointer, which moves over 
a graduated arc. The hair stretches as it grows 
damp, and contracts as it dries, the pointer at 
the same time moving either forwards or back- 
wards, its position varying with the length of 
the hair. Other hygroscopic substances, such 
as seaweed, which grows damp in wet weather 
and dries when the weather is fine, wild oats, 
whose life-like movements under the influence 
of moisture are well known, catgut, etc., are 
sometimes employed, but rather as "hygroscopes" 
(or indicators of moisture) than hygrometers 
(measurers}. 

Whilst the reading of the thermometer gives 
the temperature at the moment of observation, 
it is for many purposes desirable to know not 
only what the temperature is now, but what has 

E 



78 Weather Science 

been the highest and the lowest temperature 
during the interval since the last observation, 
the greatest heat of day and the lowest tempera- 
ture at night, etc. There are many varieties of 
instruments whereby this information may be 
obtained, but of course only a few of the more 
commonly used forms can be here mentioned. 
Instruments for recording the highest tempera- 
ture during a given interval (e.g., the course of 
a day) are called maximum thermometers, those 
giving the lowest temperature are consequently 
known as minimum thermometers. 

Of maximum thermometers perhaps the most 
generally used form is the Phillips', invented by 
the well-known geologist, though that of Negretti 
and Zambra is as good and quite as often recom- 
mended. This is a mercurial thermometer having 
a tube whose bore is narrowed close to the bulb. 
The mercury on expanding rises past this obstruc- 
tion, but when the temperature falls again, the 
liquid cannot flow back again with ease. Thus 
if the thermometer be set on any day, say in the 
morning, and examined after the temperature 
has risen and fallen again, the column of mercury 
in the tube above the constriction will represent 
the amount of liquid forced past it when the 
temperature was highest, and the end furthest 
from the bend or constriction will indicate the 
maximum temperature. Phillips' thermometer 
has a small air bubble separating a detached 
portion of mercury from the main column. 



Maximum and Minimum 



79 



When the latter expands, it pushes the bubble 
and short column before it, but when the tem- 
perature falls, though the main column contracts 
and leaves the air bubble behind, the detached 
column also contracting, but to a minute degree, 




Maximum Thermometer. 

its end furthest from the bulb indicates the highest 
temperature since the last reading. To set the 
instrument it is inclined gently, bulb downwards, 
when the air bubble becomes contracted to its 
smallest dimensions, the detached column com- 
pressing it, and coming close to the main 
portion. 

The minimum thermometer in most common 
use is that known as Rutherford's. In this 
instrument the liquid is coloured spirit (usually 




Minimum Thermometer. 



alcohol) having a metal index. This index is 
allowed to run to the end of the column by 



8o Weather Science 

inclining the thermometer with its bulb slightly 
upwards. At the same time any detached 
portion of the spirit is brought back to the 
main column by gently warming, or sometimes 
this may be done by swinging the instrument to 
and fro, bulb downwards. When this is done, 
the index being set at the top of the column, 
the thermometer is left in a horizontal position. 
If now the temperature rises, the spirit will flow 
past the index without moving it, but if the 
temperature falls, the index will be drawn back 
with the contracting fluid, for the spirit will not 
easily admit of its surface being broken by the 
index getting outside, " capillary attraction " 
keeping the index with its upper end just below 
the top of the liquid column. Thus the top of 
the index will mark the lowest temperature, since 
it recedes with a fall, but remains stationary with 
a rise of temperature. Care must be taken that 
the index is within the spirit, and that no 
detached column remains in the upper end of 
the tube. 

Though a spirit thermometer can record lower 
temperatures than a mercurial one (in this 
country of course so low a temperature as the 
freezing-point of mercury, 40 is never naturally 
reached), yet its expansion and contraction are 
less regular than is the case with the liquid 
metal, errors are frequent from the presence of 
air in the tube, from evaporation of part of the 
liquid column, etc., so that it needs frequent 



Self-recording Thermometer 81 

comparison with a standard mercurial thermom- 
eter, and even then its readings are often not 
very reliable. 

Self-recording thermometers are in use in 
various large observatories. A common method 
is to obtain a continuous automatic record by 
means of photography, in a manner similar to 
that adopted for the barometer. A sheet of 
sensitised paper is mounted on a vertical cylinder 
placed behind an upright thermometer, and is 
shielded from light by a blackened metallic 
cover, with the exception of a narrow strip just 
behind the position of the mercury. A beam of 
light from a lamp is thrown upon the apparatus, 
being stopped by the mercury in the tube; so 
that it only falls on the part of the paper above 
the liquid column. The cylinder revolving slowly 
(usually once in twenty-four hours, or sometimes 
a shorter interval), if the temperature remain the 
same, the boundary between the unaffected and 
blackened parts of the paper will be horizontal ; 
if the temperature change, which it will do con- 
tinuously, there will be seen a wavy line of 
separation, and thus the position of this will be 
a measure of the temperature at each moment. 

The photographic thermograph affords another 
method of obtaining a continuous record. Here 
there is a bubble of air on the top of the 
mercurial column which moves up and down 
with the temperature. A lamp is placed in 
front of the instrument and a photograph of 



82 Weather Science 

the position of the air bubble is taken on 
sensitised paper, which is rolled on a drum 
revolving uniformly. 

For meteorological purposes various forms of 
screens, of which Glaisher's and Stevenson's are 
the most generally used, are adopted by different 
observers. 

Whilst, on the one hand, it is necessary to 
obtain free exposure to the air, on the other 
hand, for instruments intended to give the air 
temperature, care must be taken to guard against 
direct radiation from the sun or neighbouring hot 
bodies, whereby the temperature of the thermom- 
eter may be unduly raised, whilst we must also 
see that it may not be lowered by radiation to 
colder bodies. As far as possible, the ordinary 
wet and dry bulb, as well as the maximum and 
minimum thermometers, must be surrounded by 
freely circulating air and be not too near other 
objects. 

The Stevenson screen is made of wood, " double 
louvred," the louvres sloping in opposite direc- 
tions, so that the air may freely circulate, but 
direct radiation, rain and snow are unable to 
enter. It is erected on four legs, each about 
4 feet high, and should stand on open ground 
or over grass, and not be within 10 feet of 
any wall or other building if possible, nor near 
trees. Its dimensions are usually about 2 feet 
(length), 14 inches (breadth), and 18 inches 
(height). Inside it are placed the dry and 



Stevenson Screen 



wet bulb instruments, as well as the maximum 
and minimum thermometers, the two former 
being hung vertically side by side, the two 
latter suspended 
horizontally, one 
over the other. 
Below and slightly 
to one side of the 
wet bulb instru- 
ment is placed the 
vessel of water 
(cup or small 
glass), into which 
the ends of the 
strands tied round 
the muslin sur- 
rounding the ther- 
mometer bulb, dip. 
In the Glaisher 
pattern there is a 
vertical board on 
which the thermo- 
meters hang, their 
bulbs being freely 
exposed to the air. This is attached to a sloping 
screen supported by a tripod. As the direction 
of the sun's rays changes during the day, the 
whole structure requires shifting once or twice 
daily, whilst in time of rain with northerly wind 
the instruments cannot be kept dry, as on that 
side they are merely sheltered on top and on one 




Stevenson Screen. 



8 4 



Weather Science 



Solar Radiation 

Black Bulb 
Thermometer. 



side. Thus it is in some respects less convenient 
than the Stevenson form of screen, whose double 
louvres prevent the entrance of any 
rain or snow. Special forms of instru- 
ment, such as the black bulb thermo- 
meter, which is a maximum thermo- 
meter of a particular kind, are used for 
measuring the intensity 
of radiation, its readings 
always being much higher 
than those of the other 
instruments. To the late 
Sir John Herschel, the 
well-known astronomer, is 
due the suggestion of its 
construction. It consists 
of a mercurial maximum 
thermometer, whose bulb 
and lower part of the stem 
are coated with lamp 
black. This apparatus is 
enclosed in a larger glass 
tube, one end of which 
is blown out into a large 
bulb, and the thermo- 
meter bulb rests in the 
centre of this. This cover 
has been previously ex- 
hausted of air as far as 
possible, by means of an air pump or otherwise, 
and is a partial vacuum. The whole instrument, 



Grass 

Minimum 

Thermometer. J 



Black Bulb Thermometer 85 

cover and all, is hung horizontally on a wooden 
structure at about the same height above the 
ground as that at which the other thermom- 
eters are placed (i.e., about 4 feet) in the open 
air, and at a distance from walls of buildings, 
trees, or other obstructions. The bulb is exposed 
to the full rays of the sun, and protrudes 
usually some slight distance beyond the sup- 
ports ; it is usually directed south or south- 
eastwards. 

As a 'rough rule for measuring the maximum 
radiation from the sun, it is suggested to take 
the difference between the maximum reading 
given by this instrument and the maximum 
as given by the ordinary maximum thermom- 
eter in the screen, but it is not very easy to 
attach a meaning to the number thus obtained, 
which will, moreover, differ with different ther- 
mometers, according to differences in " the coat- 
ing of the bulb, in the glass of the outer jacket, 
or finally, in the perfection of the vacuum" 
(Scott). The reading of the black bulb ther- 
mometer giving, perhaps, the highest tempera- 
ture in the sun at any moment during the day 
without reference to its duration, can afford but 
an imperfect idea of the amount of solar radia- 
tion, so that we may regard this instrument rather 
as a curiosity than as of scientific value. Its 
high readings, sometimes even in winter, may 
excite our admiration, but, just as the phenomen- 
ally low temperatures occasionally recorded by 



86 Weather Science 

grass minimum spirit thermometers, they are 
practically meaningless. 

The daily range of temperature is least in 
winter, usually not exceeding about 5 or so, 
and greatest in summer, when it amounts (in 
this country) to about 15 or 16, its value for 
spring and autumn being intermediate between 
those for summer and winter. Though the sun's 
altitude and the total amount of sunlight re- 
ceived are greatest on 21st June and least on 
21st December, the hottest and coldest times of 
the year follow about one month after these 
dates. In accordance with the old proverb, " As 
the days begin to lengthen, so the cold begins 
to strengthen," it is when the days are getting 
sensibly longer, about the end of January or 
the beginning of February, that we experience 
the coldest weather in this country. Similarly 
it is in August that the highest temperatures 
are experienced, though the day is considerably 
shorter than during June. The coldest time 
of day is usually just before sunrise, and 
the hottest time about 2 P.M. The general 
and fairly evident explanation of all these 
phenomena is essentially the same. During 
the day heat is being received in increasing 
amount from sunrise to noon, after which a 
smaller quantity comes till sunset. All the 
while heat is being radiated away from the 
earth, but at a slightly less rapid rate, and thus 
during the morning the temperature increases 



Variations in Temperature 87 

even after the amount of heat received begins to 
diminish. Thus the hottest time of the day 
is after noon. During the night no heat is 
received from the sun, and so the temperature 
continually falls, radiation going on all the while 
till sunrise. In a similar manner the amount 
of heat received each day from the sun during 
spring continually increases up to 21st June, 
but the amount radiated away, though greater 
as time goes on, does not quite keep pace 
with the increased amount received. Thus the 
temperature continues to increase even after the 
maximum daily amount of heat has been re- 
ceived, until at about the end of July the amount 
radiated equals that received daily, and the 
temperature is a maximum. After this, the 
daily amount received continuing to diminish, 
the radiation by night exceeds the daily receipt, 
and so the temperature falls. 

The mean daily temperature in this country 
will not differ much from the mean of the 
temperatures at 8 A.M. and 8 P.M., or 9 A.M. 
and 9 P.M., or if three observations are taken, 
the mean of the 9 A.M., 3 P.M., and 9 P.M. 
temperatures will give a very close approxima- 
tion to the truth. 

The mean of the maximum and minimum 
thermometers will not usually give quite so 
accurate a result, the maximum day tempera- 
ture being generally slightly more above the 
mean than the minimum is below, so that the 



88 



Weather Science 



mean of the readings of the two instruments 
will be somewhat higher than the true mean 
temperature. 

In instruments even of the best class 
occasional changes of zero, sometimes as much 
as 1 F., take place, necessitating examination 
of thermometers, and though of recent years 
great improvement in the construction of 
standard thermometers has been attained, it 
must be confessed that few instruments can 
be trusted within 0*2 or so, and this accuracy 
is perhaps more than is to be usually expected. 




The Hygrodeik. 



Measurement of Rainfall 89 



CHAPTER IV 

RAIN GAUGES SUNSHINE RECORDERS WIND INSTRUMENTS 

ANEMOMETERS DISTRIBUTION OF RAINFALL. 

THE measurement of the rainfall of any district 
is a matter in which perhaps more interest 
is taken than almost any other branch of 
meteorological science, and accordingly we find 
a very large number of stations throughout the 
British Islands where a rain gauge is installed 
and regular observations are taken daily, and 
reported to a central authority. The British 
Rainfall Organisation, founded by the late 
G. J. Symons, and now under the directorship 
of Dr H. R. Mill, publishes an annual volume, 
British Rainfall, giving the results of observa- 
tions from more than three thousand five 
hundred stations in our islands, illustrated by 
maps and diagrams, with articles upon various 
branches of rainfall work, whilst Symons 1 
Meteorological Magazine, a monthly magazine 
under the same editorship, contains records of 
rainfall and temperature at a number of stations 
each month, articles on meteorological topics, 
remarkable storms, a climatological table for 
the British Empire, and occasional contributions 



Weather Science 



on other kindred subjects, as well as letters on 
remarkable meteorological phenomena, or points 
of interest to meteorologists generally. 

There are many forms of rain gauge in use, 
from a simple upper funnel dipping into a glass 
bottle, to the most elaborate automatic self- 
recording gauge, or "pluviometer," as it is called. 
The commonest form, and one giving quite 
satisfactory results, is merely 
an improvement of Howard's 
rain gauge, so named from 
the eminent meteorologist 
whose careful study of cloud 
phenomena led him to a 
classification and nomencla- 
ture for these, now uni- 
versally adopted. A copper 
funnel, whose upper diameter 
is 5 inches, dips at its lower 
end into a glass vessel a 
wine bottle or jar with a 
neck will serve very well for 
this purpose. Sometimes 
the funnel and bottle are 
enclosed within a cylindrical copper vessel, on 
whose bottom the bottle receiving the rain rests, 
and to whose top the rim of the funnel is fitted. 
At the top, and forming part of the funnel, is a 
"vertical rim" about 6 inches in depth, and the 
inner surface of this rim fits over the metal 
cylinder. The object of the rim is to prevent 




Rain Gauge and 
Measuring Glass. 



Rain Gauge 91 

the "plashing" of raindrops and to catch any 
snow that may fall. Care should be taken that 
the mouth of the funnel is not dented, otherwise 
the full amount of rain will not be collected. It 
has been found that the diameter of 5 inches is 
the most convenient one for ordinary use, and 
though at some stations a diameter of 8 inches is 
used, and a consequent greater amount of rainfall 
may be collected, the results of experiments on 
gauges "from 3 inches to 24 inches in diameter 
show that the difference in indications is very 
small, hardly exceeding one per cent" (Scott). 
The gauge is ordinarily examined every day 
at 9 A.M., and the rain fallen in it during the 
previous twenty - four hours (if any) is poured 
into a measuring glass and measured, the amount 
being entered in the register as having fallen 
on the previous day. The measurement of snow 
or hail is usually effected by thawing this by 
means of the addition of a measured quantity 
of warm water, and subtracting the amount 
of this latter from the total resulting volume 
of liquid. Of course the snow may be melted 
by thawing it in a warm room, but this process 
is a slow one. It is sometimes said that by 
measuring the depth of the snow in a place 
where it has not drifted, and reckoning ^th 
the depth in inches, i.e., one foot of snow for 
an inch of rain, we get the amount of rain that 
would correspond to the given snowfall, but 
apart from the fact that the snow is not likely 



92 Weather Science 

to be of uniform density there is an uncertainty 
arising from evaporation, so that the result will 
only be a rough approximation. 

The height of the rim above the ground for 
a " ground gauge " should be about 1 foot ; 
the latter should be perfectly level, and to keep 
it so, as well as to prevent the whole apparatus 
being blown over by the wind the lower part 
of the cylinder should be embedded for several 
inches in the ground, leaving little more than 
the funnel and rim above the surface. A 
frequently used contrivance to diminish the 
effect of the wind as far as possible consists 
of a shield of wire-netting encircling the mouth 
of the gauge, this was proposed by Nipher in 
1878 ; the effects of evaporation are reduced to 
a minimum by making the aperture of the 
funnel which dips into the receiver as small as 
possible, and placing the latter in a vessel so 
that it is shielded from the sunlight (as in the 
outer copper vessel). It was found that rain 
gauges placed at an elevation above the ground 
catch considerably less rain than those near its 
surface, an effect now known to be due to the 
action of the wind, as it is stated that a Nipher 
shielded gauge will catch about the same amount 
on the roof of a house or other building as on 
the ground (Waldo). 

Of self-recording instruments for the measure- 
ment of rainfall a simple form is now on sale 
by various makers, and a short description of 



Sunshine Recorder 



93 



one is given in our chapter on Observatories 
(chap. xii.). 

The use of the sunshine recorder has been 
greatly extended of recent years, and now most 
seaside and watering places possess one or more 
of these instruments, and publish the results of 
these records as an attraction to possible future 
visitors. The most ordinary form of this instru- 




Sunshine Recorder. 



ment is that known as the Campbell Stokes 
recorder. This consists of a spherical lens of 
glass whose principal focus lies at a short distance 
outside its surface. It is mounted on a pedestal, 
and attached to this is a curved metal frame- 
work with three grooves, at a distance from the 
lens equal to its focal length, the whole resting 
on a slab whose surface must be horizontal ; and 
this slab should be cemented on to the surface of 
a wall of other position where it will be freely 



94 Weather Science 

exposed to the sunlight, and kept in a fixed 
position. The sun's rays passing through the 
lens are focussed at some point or other 
on the surface of the curved metal framework, 
according to the time of day and the season 
of the year, and a sheet of cardboard is fitted 
into one of the grooves. As the sun's position 
changes the position of its image changes also, 
and thus burns a trace upon the card, the latter 
being unburnt when the sun is covered by cloud. 
Owing to the sun's varying declination (and 
consequent altitude) at different times of the 
year, its image will fall higher or lower upon the 
framework, and three sets of cards of different 
sizes and shapes are provided, the smaller card 
for winter fitting into the upper groove, the 
intermediate size for spring and autumn fitting 
into the middle groove, and the longer fitting 
into the lowest groove, for summer. The instru- 
ment must be so set that the central position 
must be just opposite to that of the sun at 
noon, and its image falls upon each card at the 
point marked noon on the graduated surface of 
the latter. When the instrument is in adjust- 
ment, on a clear, sunshiny day, there will be 
formed a broad horizontal track from sunrise 
to sunset, the position of this track changing 
from day to day during the course of the year, 
as stated above, but the middle point of each 
day's trace will always coincide with the time 
of apparent noon, On a cloudy day the trace 



The Anemometer 



95 



will be fainter and often broken. It has been 
suggested to expose a photographic plate towards 
the north at night-time, and thus obtain a trace 
of the pole star on clear nights ; the record, of 
course, being broken or absent in cloudy weather, 
thus supplementing the sunshine recorder for 
the night hours. 

Instruments for wind measurements may be 
divided into two classes, those which measure 




Anemometer with Wind Vane. 



its pressure upon any surface, and those which 
record its velocity. The direction is indicated 
by a vane or weathercock, whose construction 



g6 Weather Science 

and manipulation needs little description, though 
even in this simple instrument errors occasion- 
ally arise from the confusion between true and 
magnetic bearings, the difference in this country 
amounting from 15 to 20. 

One of the most commonly used forms of 
" anemometers " for measuring the velocity of the 
wind is that invented by the late Dr Robinson 




Robinson's Anemometer. 

of the Armagh Observatory, and known by his 
name. It consists essentially of four hemi- 
spherical cups which are fixed at the ends of 
two horizontal rods forming a cross. These are 
fastened to a vertical axis, the latter passing 
through the point of intersection of the rods. 
The wind causes these rods or arms to revolve, 
and they are so constructed that a definite rate 
of revolution corresponds to a definite wind 



Different Forms of Anemometer 97 

velocity. The motion is registered by a wheel- 
work arrangement at the base of the apparatus, 
being communicated to a system of dials which 
record the number of revolutions made, whence 
the speed of the wind may be inferred. There 
is sometimes an arrangement for marking the 
numbers of miles of wind on a paper strip 
fastened to a drum moving regularly by clock- 
work. 

As an instrument for recording pressure, Lind's 
anemometer may be mentioned. This consists 
essentially of a glass syphon, one end closed, the 
other end bent and open to the wind. The 
limbs of the syphon are vertical, the open end 
is horizontal. Water is poured in up to a given 
height, standing at the same level in both legs 
when the air is calm. When the wind blows, 
the level will be more or less depressed in the 
one, and rises correspondingly in the other to 
a greater or less degree according to its force. 

Dines' pressure anemometer is a more exact 
instrument, but depends on a similar principle. 

Osier's and Gator's anemometers are more 
complicated instruments. In these the pressure 
against a plate of given area is measured, whilst 
a resistance is afforded by springs or by weights. 

Wild's pressure gauge consists of a rectangular 
plate, hung on hinges to a horizontal axis. The 
angle made by this with the vertical measures 
the force of the wind. Its indications, though 
sufficiently accurate for light winds, do not show 



98 Weather Science 

the differences between those of strong force 
(Scott). 

The distribution of rainfall throughout the 
world is a subject of considerable importance 
from the economic point of view as well as 
the meteorological one, for the habitability of 
any district is largely dependent upon this factor. 
Even in our own islands the local variations 
are very considerable, there being a mean annual 
rainfall of over 160 inches in the lake district of 
Cumberland, whilst in Lincolnshire the annual 
precipitation does not usually much exceed 
20 inches. Similarly in India, Cherrapunji, on 
the Khasia hills, has an annual rainfall of 
500 inches, whilst in the Deccan there are frequent 
droughts, and the total amount is far below 
the average. As a very rough approximation 
it is sometimes stated that the annual rainfall 
at any place within the Torrid Zone is about 
100 inches, the Temperate Zone has 30 inches, 
and the Frigid Zone 15 inches. Nevertheless, 
though it is probably true that as a general rule 
more rain falls over the Torrid Zone than else- 
where, there are also to be found over the same 
zone the driest regions of the globe, the Sahara 
and Arabian deserts, and the great desert of Gobi 
or Shamo. In South Africa the driest region 
is probably the Kalahari desert, whilst in 
America there are the " nitrate " regions of Peru 
and Northern Chili, and the great Salt Lake 
district of the United States. The rainfall of 



Agencies affecting Rainfall 99 

Australia is in general small, and throughout 
the greater part of the interior of the island 
continent there falls considerably less than 
10 inches per annum. 

There are three principal agencies which are 
most efficacious in bringing about a fall of 
rain: (1) mixture of masses of air at different 
temperatures, (2) ascending currents of air, (3) 
contact of warm air with the cooler surface of 
the ground. Of these three agencies, probably 
the second ascending currents of air, which 
by expanding fall in temperature is the most 
commonly acting, at least within the tropics. 
It has been calculated that the temperature of 
a mass of dry air will fall about 1 C. for every 
100 metres of ascent (Hann). If, however, the 
air, as is always the case, contains more or less 
moisture, the rate of cooling will be consider- 
ably less, since condensation, converting some 
of this vapour into the liquid form, sets free 
" latent heat," which tends to warm the sur- 
rounding air. Nevertheless, the fall of tempera- 
ture produced by a rise from the surface of the 
ground to the top of a mountain is very con- 
siderable. The heated warm air over the surface 
of the ocean always containing much vapour in 
suspension, rising to a greater or less height, 
sooner or later loses most of its moisture, 
which falls in the form of rain. Near the 
Equator, generally speaking, there is an almost 
continual precipitation in the form of heavy 



too Weather Science 

showers, and the fall is greatest when the 
weather is hottest. 

The contact of warm damp air with the colder 
surface of the ground is the main cause of the 
greater rainfall on the western part of these 
islands (Great Britain and Ireland) as compared 
with the eastern. The prevalent wind being 
south-westerly, which has blown over more or 
less of the Atlantic before reaching us, arrives, 
nearly saturated with moisture, first upon the 
hilly western portions of the country. The air, 
being forced to ascend, is cooled by contact 
with the hilltops, and thus we get the heavy 
rainfall of our western counties, which, though 
they are thus wetter, have also a milder tem- 
perature than the inland and eastern parts of 
the country, (partly) through the " latent heat " 
set free during the process of condensation. 
The third method of condensation mixture of 
masses of air at different temperatures though 
considered by some authorities as important, is 
estimated by Dr Hann as of minor value as a 
cause of precipitation. 

The distribution of rainfall throughout the 
globe, though in general, as already stated, de- 
creasing from the Equator polewards, is subject 
to great local variation. In tropical regions 
the year is usually divided into the " dry " and 
" rainy " seasons respectively. Thus at Panama 
the rainy reason is from May to November, the 
dry season from November to May. In regions 



"Dry 1 ' and "Rainy" Seasons'' 'ioi' 

where the trade winds blow there is but little 
rain during the time of their occurrence, whilst 
the descent of the return trades brings abundance 
of rain. Since these regions, approximately 
30 to 40 N. and S. latitudes, have their rain 
when the sun is lowest, they may be called 
the regions of the " winter rains," in contra- 
distinction to those regions where most rain falls 
when the sun is at its highest. 

Most of the countries round the Mediterranean, 
California in America, Cape Colony, etc., come 
under the former category. On the other hand, 
in Natal, the Argentine Republic, China, and the 
Eastern United States, more rain falls during 
the summer than during the winter periods 
of the year. In our own islands, though the 
total amount of rainfall is less than in tropical 
and sub-tropical regions generally, there is not 
the same seasonal distinction, though there is, 
perhaps, a greater amount of precipitation during 
the winter than the summer months ; yet the 
difference is not a large fraction of the whole 
amount, so that we may say that we have " rain 
at all seasons." 

It is a general rule that the heaviest rain- 
falls occur in the western counties in Ireland, 
Scotland, and England alike, partly owing to 
the fact that the most rain-bearing winds coming 
from the south-west reach these regions earlier, 
and partly owing to the mountainous nature 
of the western districts, the cooling of the 



102 



Weather Science 



air currents by passing over them producing 
a deposition of most of the moisture. Maps 
showing the total rainfall at several thousand 
stations throughout the United Kingdom, are 
published annually by the British Rainfall 
Organisation, founded by the late G. J. Symons, 
and now under the direction of Dr H. R. Mill. 

Note. By the term annual rainfall, so many inches, is meant the 
depth of water that would be obtained if all the rain which falls 
there in a year were collected into one horizontal sheet, and none 
were lost by evaporation or absorption into the soil. A 5-inch 
gauge, the area of whose surface consequently 

=i T d 2 = (3-14]6)(25) = 19'63 square inches 

thus collects nearly twenty times as much rain as falls upon each 
square inch in its vicinity, and so enables measurement of rainfall 
to the nearest hundredth of an inch (or even less) to be made. One 
inch of rain means that if all the rain falling over any given area 
were spread out uniformly, it would form a layer having that depth. 




Self-Recording Rain Gauge. 




60 



oo 



MEAN ANNUAL RAINFALL MAP OF THE 

BRITISH ISLES. 
(By kind permission of Dr. H. R. Mill.) 

Annual Rainfall. 

Below 25 in Blank 

,, 25-30 ,, ... ... ... Dots 

30-40 ,, ... ... ... Faint shading 

40-60 Darker shading 

Above 60 Black 



To face p. 102. 



Weather Forecasts 103 



CHAPTER V 

WEATHER FORECASTS TYPES OP CIRCULATION THE SEVEN FUNDA- 
MENTAL FORMS OF ISOBARS CYCLONES SECONDARIES V-SHAPED 

DEPRESSIONS COLS STRAIGHT ISOBARS ANTICYCLONES WEDGES 

LOWER AND UPPER WINDS. 

FROM the earliest times inferences as to the 
probable future course of the weather have been 
drawn with greater or less certainty from the 
state of cloudiness, direction of winds, halos, 
and other optical phenomena, and many " prog- 
nostics " are well known and widely prevalent. 
A very complete collection of these " saws and 
sayings" is given by Mr Inwards in his 
" Weather Lore," and though many are little 
warranted by the actual facts of the case, on 
the other hand many are of great value, and 
" in conjunction with other aids to weather fore- 
casting, prognostics will never be entirely 
superseded, especially for use on board ship" 
(Abercromby). 

The invention and use of the barometer has 
led to a fresh set of prognostics, and there are 
still many persons who have great faith in the 
legends "fine," "rain," "set fair," etc., still to 
be found on the " weather glass," though these 



IO4 Weather Science 

terms are only correct in the broadest and most 
general sense, and are often without meaning. 
All such indications and prognostics, however, 
are of a very uncertain character, and their 
frequent failures are a subject of common remark. 
Even as it is now, with the immense mass of 
statistical information, records of temperature 
pressure, rainfall, etc., all over the habitable globe, 
we are in little better position to issue reliable 
forecasts. The average temperature is thoroughly 
well known, but no one can say what the actual 
temperature at any hour throughout the year 
will be; the total amount of rainfall has been 
measured at some stations for two centuries, 
but no human being can say with certainty 
what amount will fall at any given place four 
days hence. 

Occasional attempts to predict the general 
course of the weather for months ahead may 
be one and all set down as premature, nay, in 
some cases they are little better than the char- 
latanry of the fortune-tellers and astrologers. 
Such knowledge as we have yet gained is of 
a purely general and statistical character, lead- 
ing us to mean and average results, but in no 
case to inferences for individual future times. 
The weather forecasts issued by our own and 
foreign meteorological offices are often remark- 
ably accurate, but are necessarily of the most 
general character, and can only be issued a day 
or so ahead, though in times of fairly settled 



Fundamental Forms of Isobars 105 

weather a good idea may be gained of its course 
for the next three or four days. 

This forecasting by means of " synoptic charts," 
and the information as to the conditions pre- 
vailing in surrounding districts, obtained and 
telegraphed to a central office, has already been 
alluded to, but we propose here to give a some- 
what more detailed account of this branch of 
experimental meteorology, a science (if it may be 
called so) as yet only in its infancy. The seven 
fundamental forms of isobars, already referred 
to, are the cyclones, anticyclones, secondary 
cyclones, V-shaped depressions, cols, wedges, 
and straight isobars. The cyclones, secondaries, 
and V's enclose areas of lower pressure, the 
centres being at a pressure more or less below 
the average ; the anticyclones, wedges, and cols 
enclose regions of higher pressure. It has been 
found that as a rule cyclones, secondaries, and 
wedges move eastwards and north-eastwards in 
our latitudes, whilst anticyclones are often 
stationary for days or weeks, and occasionally 
even for months at a time, breaking up rather 
than moving on. 

A cyclone may be defined as a "large disc 
of nearly horizontally moving air circulating 
spirally round a central area over which the 
barometric pressure varies from one-fifth to as 
much as three inches (of mercury) below that 
at its border" (Archibald). The diameter of a 
cyclone varies from 20 to 2,000 or even 3,000 



io6 Weather Science 

miles, and the form is more often oval than 
circular, the isobars being usually not quite 
concentric. Two cyclones differing from one 
another in the greater or less closeness of the 
isobars, the general character of the weather 
experienced during their passage will be the 
same, but the wind will be stronger in the 
former than in the latter case. The main 
difference between the tropical cyclones causing 
storms and the cyclones of temperate regions 
consists in this point. 

Tropical cyclones are usually small and move 
forward at the rate of from 2 to 10 miles per 
hour (Abercromby), but the wind round them 
moves at a very high speed, perhaps 100 miles 
per hour. Those of temperate regions, on the 
other hand, have quicker translatory movement, 
perhaps from 20 to 50 miles per hour, but their 
rotatory speed (and consequent wind) is much 
slower. Thus we may consider the " rotatory " 
phenomena as due to the circulation, the " trans - 
lational " to the forward movement of a cyclone. 
In Europe and North America the usual course 
of a cyclone is towards the east, but in the 
northern tropics hurricanes move towards the 
west. Occasionally, even in our latitudes, a 
cyclone has a westerly motion, and then the usual 
prognostics and weather signs are said to " fail." 

The usual symptoms of the approach of a 
cyclone in our latitude are as follows (apart 
from the fall of the barometer, usually accom- 



Signs of a Cyclone 107 

panied by the rise of the thermometer) : The 
air has a close, "muggy" feel, drains begin to 
smell, persons subject to rheumatism complain 
of their pains, cirro-stratus and cirrus clouds 
gradually cover the sky, the sun and moon 
when seen low down are pale and "watery," 
frequently surrounded by halos. As the centre 
approaches, light showers or drizzling rain begins 
to fall, the wind blows in gusts, and usually 
from the south-east or south - west. After a 
while the barometer ceases to fall, and a short 
bright interval, followed by squally showers, 
succeeds. The centre is now passed, and the air 
assumes a brisk, exhilarating "feel." A hard 
sky with detached masses of clouds of the 
cumulus type characterises the rear of the 
cyclone, just as the front is characterised by 
cirrus or cirro-stratus. The north-west wind 
"improves men's tempers as opposed to the 
neuralgic and rheumatic sensations of the front." 
Secondary cyclones are small depressions 
usually associated with larger cyclones, though 
also found at the edges of anticyclones. The 
wind usually blows in angry gusts, not in the 
steady, regular manner of " cyclone wind." The 
motion of a secondary is generally parallel to 
that of the primary. Secondaries are frequent 
indications of rain without much wind. Their 
sudden formation frequently produces the falsi- 
fication of previously made forecasts. Contrary 
to the general rule of rain with a falling barom- 



io8 Weather Science 

eter, heavy rain, at first in gusts and then more 
steadily, sometimes for several hours continually, 
with steady or even slightly rising barometer, 
accompanies the passage of a secondary. 

Allied to secondaries, we have the form 
of isobars known as V-shaped depressions, the 
isobars enclosing an area of low pressure, taking 
a shape like the letter V. These often occur 
along the southern prolongation of a cyclone, 
and the point of the V is usually directed south- 
wards in our hemisphere, or else lies in the 
"col" or region of low pressure between two 
adjacent anticyclones. Two distinct types of 
V's are usually enumerated. In the first and 
most common kind "a narrow strip of cloud pre- 
cedes an area of rain, followed by detached 
clouds and blue sky." In the second kind we 
have the front cloudy, and "half a crescent- 
shaped area of rain in the rear." The trough 
of the depression marks off the front of this 
area, but the rear is ill defined. An example 
of this kind of circulation bringing with it a 
"line squall" caused the capsizing of H.M.S. 
Eurydice off the Isle of Wight in 1878. The 
usual sequence of weather, as this form passes 
over a station, is from blue sky to cloud and 
wind from the south-west, with falling barometer. 
Then a heavy bank of cloud comes from the 
north-west, passing over with a squall; the 
wind suddenly changes to north-west, and the 
barometer rises. After the squall, driving rain 



Cols, Straight Isobars 109 

continues for some time, gradually ceasing, and 
the sky clearing once more. Many of the 
"southerly bursters" of Australia are said to 
belong to the class of V's in which rain falls in 
rear of the trough ; the point of the V is here 
turned northwards, whilst the wind is north-east 
in front, and south-west or south in the rear. 

The col, as just mentioned, is a neck of low 
pressure lying between two anticyclones. In 
the middle there is usually no wind, whilst on 
the edges the wind blows according to the 
usual rule of isobars. The weather is generally 
dull and gloomy, and frequently violent thunder- 
storms occur (in summer) during the prevalence 
of these conditions. The col does not itself 
move, and no regular sequence of weather can 
be assigned to it. 

Straight isobars are sometimes found near the 
northern edges of anticyclones. They rarely 
persist for long, and are soon followed by a 
cyclonic depression. They thus usually indicate 
cloudy, unsettled weather, with some wind, soon 
to be succeeded by more or less rain, though 
the air is drier than in the case of cyclones. 
"Visibility," "the sun drawing water," and 
"audibility" frequently occur as accompanying 
this form of isobars. 

In every respect contrary to the cyclone is 
the next most frequent type of circulation, called, 
from its general opposition in properties, the 

"anticyclone." 

G 



no Weather Science 

In this the isobars are usually much wider 
apart, than is the case with those composing the 
cyclone, and the gradient of pressure is upwards 
towards the centre, diminishing gradually towards 
the outer portions. Whilst the cyclone is usually 
in fairly rapid motion, the motion of an anti- 
cyclone is very slow, and sometimes the system 
remains almost stationary for days or weeks at 
a time, finally breaking up and being replaced 
by a cyclone. The circulation round the centre 
is "clockwise," but the wind is much less in 
force than for cyclonic disturbances, as is shown 
by the greater distance apart of the isobars. 
During the prevalence of an anticyclone we 
have in summer, blue sky, hot sun, and little 
wind; in winter, frost and fog, and sometimes 
biting east winds, with gloomy black sky. In 
the centre of the anticyclone there is a dead 
calm, whilst the circumferential winds are 
usually slight and centrifugal, or slightly curved 
outwards ; the wind in a cyclone being centri- 
petal or incurved. Extreme dryness, accom- 
panied by heat in summer (though sometimes 
slight showers fall), and cold, frosty (sometimes 
also foggy) weather in winter, are the constant 
characteristics of this type of circulation. Anti- 
cyclones are the most persistent types of atmos- 
pheric circulation. 

Wedge-shaped isobars are projecting areas 
of high pressure moving between two cyclones, 
and may be regarded as the converse of V 



Wedges 1 1 1 

depressions, just as anticyclones are the con- 
verse of cyclones. The wedge may point in 
any direction, but most commonly is directed 
towards the north. On the front or eastern 
side the weather is bright and the sky clear, 
the wind being of moderate force, and blows 
round according to the general law of gradients 
(from the north-west on the east side) ; in the 
centre there is a calm, and on the west side the 
wind is south-west, the sky becomes overcast, 
usually with cirro-stratus clouds, and next comes 
rain from the following cyclone. 

Thus the sequence of weather, when the wedge 
travels eastwards, is fine, with north-west wind 
and rising barometer ; then calm and mist or 
fog ; then halo and gloomy sky, falling barometer 
and rain, with south-west wind from the new 
cyclone. Thus the prognostics of halos, strips of 
cirrus, known popularly as " Noah's Ark," etc. 

It is to be remarked that all cyclones are 
not preceded by a wedge, but "only those 
which roll, as it were, along the northern edge 
of large stationary anticyclones " ( Abercromby). 
Appearances of the sky characterising the front 
of a wedge are signs of coming rain, of the 
very opposite kind to those of the rain prog- 
nostics of a cyclone. 

Thus in a very broad, general sense we may 
consider cyclones, V-shaped depressions, straight 
isobars, and cols as indicative of unsettled condi- 
tions ; anticyclones and, to a less degree, wedges, 



H2 Weather Science 

high-pressure areas of fine weather, the former 
of a more or less permanent character, the latter 
transient and " too fine to last." We may dis- 
tinguish between the rain of a cyclone, which 
is heralded by great dampness of the air, the 
rain following wedges, due also to the succession 
of a cyclone, though the air in the wedge is 
itself dry, and the slight showers associated with 
straight isobars. 

We have just given some account of the 
course of circulation, direction of winds, etc., 
in these various types of isobars, and must 
now proceed to the consideration of the upper 
currents associated with these. 

The surface winds of the cyclone may be 
described as moving in an " ingoing spiral," more 
incurved in the right front than elsewhere, and 
less incurved as we approach the centre, than 
in the regions outwards. 

The upper currents, on the other hand, blow 
outwards in front (in the fore part of the 
cyclone), their direction makes a considerable 
angle with that of the lower currents, but at the 
rear they are more nearly parallel to the latter. 
For the anticyclone also, the surface winds and 
the upper currents are even more opposed in 
direction than is the case for cyclones; the 
former blow spirally outwards in the clockwise 
direction, the latter inwards. In every case the 
upper currents move more quickly than those 
near the surface; this is, no doubt, partly due 



Surface and Upper Winds 113 

to the decrease of friction, just as winds over 
the surface of the sea blow more strongly than 
on the neighbouring lands. The increase of 
speed upwards, from observations made first by 
Stevenson, of Edinburgh, later by Professor 
Archibald, and more recently by the observers 
at Blue Hill and elsewhere, appears to be very 
considerable. Whilst the surface winds experi- 
ence the full effects of friction with the ground, 
at a very small altitude this effect rapidly 
diminishes. Professor Archibald found that the 
average velocity at 1,600 feet is just double that at 
100 feet, and still continues to increase for greater 
heights. Clayton's observations at Blue Hill give 
for the level of the stratus clouds (1,670 feet), 
an average speed of 19 miles per hour, increasing 
to 24 at 5,326 feet (height of the cumulus), and 
velocities of 71 and 78 miles per hour respec- 
tively, for altitudes of 22,000 feet (cirro-cumulus), 
and 29,000 feet (cirrus). In winter the speeds are 
greater than for summer, and speeds of the 
upper cirrus clouds amounting to as much as 
96 miles per hour are recorded. The speeds 
noted in European localities for the upper air 
seem to have usually somewhat less than these 
values. (See also chap vii.) 



CHAPTER VI 



MORE DETAILED DESCRIPTION OF COURSE OP CIRCULATION - WEATHER 
IN (1) CYCLONES, (2) ANTICYCLONES - SPECIAL VARIETIES WHIRL- 
WINDS - TORNADOES. 



THE name " Cyclone " (from the Greek /c 
a circle), though popularly applied only to the 
violent storms of the tropics (the air in which 
moves round and inwards towards a central 
region, hence the name), is applied by meteor- 
ologists to any portion of air moving in such a 
manner, the pressure at the centre being lower 
than that at the borders ; the difference between 
one cyclone and another being (1) a difference 
in size ; (2) difference in shape, and closeness of 
the isobaric lines. It is upon this latter that 
the force of the wind depends, as we have 
already seen ; the ordinary cyclones of (usually 
unsettled) weather, and those which cause violent 
storms, differ mainly in this respect ; but the 
latter are also usually much smaller in size 
than the former. 

In our own country, in addition to this circu- 
latory or spiral motion of cyclones, they have 
usually a motion of translation from south-west 
towards north-east, though they are sometimes 

114 



Weather Signs of Cyclone 115 

stationary ; at other times they move in the 
opposite direction, and occasionally fill up with- 
out moving on. 

The weather signs at the front of a cyclone 
are usually those already mentioned, gloomy, 
close, and muggy weather, drains smelling offen- 
sively, the sun and moon seen dimly surrounded 
by halos, and the cloud formations of a cirro- 
stratiform type. If now the cyclone remains 
stationary, or dies out, or moves in a different 
direction, then these prognostics of coming bad 
weather would be said to fail. During the 
course of a cyclone travelling more or less 
centrally across any district, we find the follow- 
ing accompaniment of weather, cloud, etc., 
and various popular sayings are associated with 
different phases. The first signs of approach 
of a cyclone, "the front," is often heralded by 
halos, seen round the sun and moon, commonly 
when they are low down in the south-west. 
Next follows denser clouds, giving rise to the 
" watery sun," seen dimly through their greater 
thickness. After this follows rain, at first slight, 
but soon more continuous and heavy, accom- 
panied by driving wind. All this time the 
barometer is falling continuously with greater or 
less rapidity. The wind at first (assuming the 
the usual direction of a cyclone in this country, 
from south-west towards north-east, roughly 
speaking), is from the south-east, and moderate 
in force, but changes in direction and increases 



u6 Weather Science 

in force, as the depression advances; veering 
towards the south, and then becoming south- 
westerly, perhaps increasing in force to a gale. 
Now the barometer will begin to rise again, the 
centre line or trough having passed, and the 
wind suddenly veers round, or rather "jumps" 
towards the west or west-north-west, its force 
being much greater than has hitherto been the 
case, rain coming on more heavily than ever. 
This ceases after a while, the barometer con- 
tinuing to rise, and patches of blue appear in the 
sky with " rocky " cumulus cloud and moderate 
wind. 

Lastly, after a few "clearing" showers, the 
wind falls to a gentle breeze, the heavens 
becoming clear and cloudless, and the cyclone 
has passed. When the observer is situated ex- 
actly on the path of the cyclone centre the wind 
"jumps " or changes when the centre passes with- 
out " veering " or " backing " through the inter- 
mediate directions, from south-west to north- 
east; if he, as is usually the case, is to the south of 
the centre, the wind " veers," very rarely for this 
country, when the cyclone centre is to his south- 
ward the wind " backs." If the wind changes 
with the " sun," i.e. 9 from east by south to west, 
or in the same direction as the apparent motion 
of the heavenly bodies in the sky, this is called 
"veering"; if its change is in the opposite direction, 
i.e., from westwards by south to east, or from east 
by north to west, it is said to " back " or change 



"Veering" and "Backing" of Wind 117 

against the sun. A change of direction, such as 
that from south-west to north-east, without either 
veering or backing, such as occurs after the centre 
of a cyclone has passed directly over the observer's 
position, is conveniently called a "jump." Mr 
Abercromby quotes the following prognostic 
with reference to the " backing " of the wind : 

" When the wind veers against the sun 
Trust it not, for back 'twill run." 

This he explains by the usual sequence of dis- 
turbance. When in Northern Europe a cyclone 
passes to the south (a rare phenomenon), thus 
producing the backing of the wind against the 
sun referred to, it is almost always followed by 
another cyclone to the north, which brings more 
bad weather and fresh changes of the wind. 

From the general indications given by clouds, 
wind, and barometer as outlined above, the 
observer can easily ascertain his position with 
regard to the disturbance, and its general course. 
The various directions of the wind in the 
different portions of a cyclonic disturbance are 
related to the pressure (in different parts) by 
the "law" of Buy Ballot, given more fully in 
our chapter on the Winds (viii), sometimes 
enunciated thus : 

"Stand with your hands stretched out on 
either side and your back to the wind, then, 
in the Northern Hemisphere the centre of the 
cyclone will be to your left hand ; if you are in 



ii8 Weather Science 

the Southern Hemisphere the centre will be to 
your right." 

This form, however, ignores the incurvature of 
the wind, which, instead of blowing directly along 
the isobars, makes a considerable angle therewith ; 
this varies with position on the surface of the 
globe, being also different for storms on land 
and on sea. The " inclination " was found to be 
62 for the Philippine Islands (latitude 14) ; in 
Bengal (latitude 20) it was 57 ; over the 
Atlantic 30 ; in England only about 20 ; thus 
apparently greater for equatorial regions than 
for stations further northwards. From this it 
follows that the old law of storms, which 
supposed the wind to " blow in a circle " directly 
along the isobaric lines, is quite unreliable for 
tropical latitudes, where the deviation is often 
considerable. Thus it sometimes happened that 
by following this rule, in the old sailing-ship 
days, a captain might sometimes run his ship 
directly into danger when seeking to avoid it. 
Whilst cyclones form, perhaps, the most pro- 
minent feature of the atmospheric circulation in 
our latitudes, they are less common in the 
tropics, though of a more dangerous type, as 
has been mentioned, smaller but with much 
greater and more violent wind motions, and it 
is from these that the word "cyclone" has 
acquired its popular meaning of a fearful storm. 

The researches of Piddington, Redfield, and 



Meldrum on Cyclones 119 

Dove on Indian, American, and European 
cyclones respectively, established the true form 
of the movements of wind, that the latter circu- 
lates in a spiral curve round the centre or point 
of lowest pressure. 

The late Dr Meldrum, for many years 
Director of the Royal Alfred Observatory and 
Government Meteorologist at Mauritius (whose 
researches on cyclones have recently been utilised 
by Mr Maunder of Greenwich to point out an 
unexpected relation between their sequence in 
tropical latitudes and the sun's rotation period), 
by his work largely helped to develop a more 
satisfactory set of rules than those afforded by 
the " circular theory." Even so, all that can be 
done is to give rough general rules, for, as 
pointed out by Dr Meldrum, in some cases the 
wind, instead of blowing at right angles to the 
radius, blew directly towards the centre (Scott). 
The modern rules now advise the mariner " to 
avoid" running before the wind, to lie to on 
the starboard tack (i.e., with the wind on his 
right) in the Northern Hemisphere, or on the 
port tack (wind on left) in the Southern. 

Tropical cyclones occur more frequently in 
September and October for the Northern Hemi- 
sphere; in February and March, the "cyclone 
seasons," for the Southern ; or more generally in 
the summer and autumn of both hemispheres. 
Off the Indian coasts they are most common 
and dangerous at the changes of the monsoons 



I2O Weather Science 

during May and October respectively. At times 
there is a difference of not less than 2 inches 
of pressure (mercurial barometer) between the 
centre and outer circumference of a cyclone. 
During one storm the barometer at Marie- 
Galante in Guadaloupe fell from 29-646 inches 
to 27*953 inches in the course of seventy 
minutes, between 6.30 and 7.40 A.M. on 6th 
September 1865 (Buchan). 

It sometimes happens that such a cyclone 
will travel from the tropics northwards towards 
Europe and more temperate regions, where it 
will become a much milder phenomenon. This 
is another proof that the ordinary storms of 
Europe are phenomena of the same nature 
as hurricanes, though differing from them not 
only in intensity, but probably in the shape of 
the storm area, and also apparently in the 
unequal development of the wind from the 
different points of the compass (Scott). 

Calculations have been made by the curious 
as to the amount of energy developed in some 
of these disturbances. For instance, Professor 
Reye of America calculated that the Cuban 
cyclone of 5th October 1844 used up 473,000,000 
units of horse-power in three days. 

As a theory of the cause and movement of 
cyclones, a brief outline of Ferrel's views may 
be of interest. Ferrel, sometimes called the 
" Newton " or the " Father of Modern Meteor- 
ology," originally an elementary school teacher, 



Fen-el's Theory 121 

who by his original researches and other work 
has so largely added to our knowledge, con- 
structed a consistent theory of these phenomena. 

By assuming an inflow towards and an upflow 
over a given area, he showed that (neglecting 
friction) the air would tend to rotate round a 
central area, at the inner portions of which the 
pressure would be very low, but this would 
gradually increase from the centre outwards, the 
whole moving in a direct or counter clockwise 
direction. Outside of this area there would be 
another area moving in the opposite or clock- 
wise direction. The interior region would thus 
be the cyclone ; the outer a kind of anticyclone 
or pericyclone. The effect of friction would 
be to somewhat modify this ideal state of affairs ; 
the pressure near the centre would be low, but 
not quite so low as would otherwise be the 
case, the (interior) motion of the inner portions 
being somewhat more "centripetal" (towards 
the centre), that of the outer part "centri- 
fugal" (outwards). 

The clear, calm region usually only a few 
miles in extent, at the centre of a tropical 
cyclone, sometimes called the "eye" of the 
storm, was explained by him. 

Some modern "specialists" consider that 
extra tropical cyclones, at least in part, are due 
to somewhat different causes from those of the 
tropics. It is well known that stormy con- 
ditions, heavy rain, etc., are, on the whole, more 



122 Weather Science 

prevalent in winter than in summer, and it has 
been thought that they are due to eddies or 
whirls in the upper "return currents" flowing 
over from the Equator, which are crowded into 
the " narrower latitudes." These eddies cause the 
lower air nearest the surface to ascend ; this air 
forms clouds, whence sometimes rain falls, but 
the movements are less violent near the ground 
than in the upper regions. 

The general direction of motion of cyclones 
eastward in these latitudes has already been 
alluded to. They appear to be governed chiefly 
by the prevalent west wind, both upper and 
under, and are carried along like eddies by the 
current. They often exhibit a remarkable 
tendency to follow the same course, several 
successive depressions quickly succeeding one 
another. Thus during weather of the "westerly" 
type, in Great Britain, when the depressions are 
so far south as to cross that island, the centres 
have a decided tendency to traverse either the 
line of the Caledonian Canal in Scotland, or the 
low-lying ground which separates the valleys 
of the Forth and Clyde (Abercromby.) Other 
cyclones coming in from the Atlantic often 
hug the coast of Norway instead of going 
north-east. 

Mountain chains also powerfully influence 
their direction. For instance, the Alps forms 
a natural boundary between the Mediterranean 
weather and that prevailing in the more northern 



Bebber on Cyclones 123 

parts of Europe ; the Himalayas, to the north 
of India, also, even more powerfully influence 
the weather conditions of that great country. 
This tendency of cyclones to follow certain 
definite directions is, of course, of great im- 
portance in connection with forecasts as to their 
probable course and duration. The influence 
of heat in determining their course has also 
been considered by Dr Bebber and others. The 
former has enunciated the following relations, 
especially for Central Europe : 

"When the distribution of air pressure and 
temperature are in the same sense, then the 
depression is propagated nearly in a direction 
perpendicular to the temperature and pressure 
gradients ; if they are distributed in the opposite 
sense the motion of the depression ceases or is 
checked." 

He considers pressure to be the more im- 
portant factor in determining cyclone motion in 
winter, whilst in summer, temperature difference 
is the predominant influence. With regard 
to the "sense" of temperature and pressure 
gradients, the following definition is given : 

" If the highest pressure and highest tempera- 
ture are both to the north, or both to the south 
of a cyclone, they are said to be in the same 
sense (and the depression will move at right 
angles to both). But suppose pressure was 
highest to north, and temperature highest to 
south, then these two elements are said to be 



124 Weather Science 

distributed in the opposite sense, and the cyclone 
would probably be arrested in its usual eastward 
course" (Abercromby). 

Cyclones frequently exhibit a tendency to 
move round anticyclones usually in such a 
manner as to keep the anticyclones on their 
right in this hemisphere, whilst occasionally 
they appear to move round one another, or again 
the phenomenon of two cyclones moving round 
a common centre is presented. 

As already stated, whilst the surface currents 
in a cyclone move after the manner of an ingoing 
spiral, but with their direction less incurved 
towards the centre, in the upper parts of the 
disturbance, at heights above 10,000 feet from 
the earth, the wind blows in a more irregular 
spiral outwards, being in front very much inclined 
outwards, but in the rear nearly parallel to the 
lower surface currents. At intermediate heights 
the course of the wind is considered to be nearly 
parallel to the isobars, or moving almost in a 
circular, or more strictly oval, manner, since 
few cyclones are even approximately circular, 
the late Professor Loomis stating that in the 
United States a circular "cyclone" does not 
occur more than once during the course of the 
year, the average ratio between the longest and 
shortest diameters being about 1-94 to 1, or 
roughly, rather less than 2 to 1. We have 
already detailed the general distribution of 



Size of Anticyclones 125 

cloudiness in a cyclone, but the actual circum- 
stances vary somewhat in different cases. The 
most common occurrence of rain with low 
pressure does not, however, prevent occasional 
instances of an exception to the general rule 
being sometimes perceived. In some cases well- 
marked and large depressions have been formed, 
but the barometric reading at the centre was 
only slightly less than on the outside, perhaps 
not lower than 297 inches (of mercury), the 
gradient is consequently very small, with light 
winds, small and slow fluctuations of pressure ; 
unaccompanied by rainfall, or at most very 
slight showers have fallen. 

We next come to consider the large often 
stationary areas of high pressure from which the 
air often flows outwards, to feed the cyclones, 
whose characteristics are in so many respects 
opposite to those of the latter that the name 
of anticyclone has been universally applied to 
them. Anticyclones are generally of much 
larger size than cyclones, and sometimes cover 
a whole continent, or extend half over the 
ocean, and are very persistent phenomena. The 
pressure is highest at the centre and the isobars 
are more nearly circular than those surrounding 
the cyclone centre, and much wider apart ; in 
consequence there is usually but little wind any- 
where. There is practically a calm in the central 
portion, but on the outer parts of the system 
the wind blows round the centre in the clock- 

H 



126 Weather Science 

wise direction. Like the wind of cyclones the 
surface wind direction is not along or parallel 
to the isobars, but spirally outwards. Just as 
with depressions the surface and the upper winds 
are opposed in direction, the upper currents 
blowing spirally inwards, their direction being 
more inclined to the isobars than the lower. 
The whole system, though often at rest and 
persisting till it breaks up, occasionally moves 
slowly along, usually in a direction east to west, 
or sometimes north-west to south-east. 

During the summer-time this type favours dry, 
hot weather, in winter east wind and overcast 
conditions. Often the morning is somewhat 
hazy or foggy, but this mistiness is usually 
dispersed by the power of the sun's rays ; the 
broad features of anticyclone weather being put 
as blue sky, dry cold air, hot sun, and hazy 
horizon, with very little wind, the type called by 
Abercromby "radiation weather." In summer, 
mist in the morning and evening, with fine, hot, 
cloudless day ; in winter similar conditions, but 
more pronounced fog, and often instead of a 
clear sky, overcast, gloomy, rainless weather, 
sometimes accompanied by the well-known east 
wind whose presence is so unwelcome: 

" When the wind is in the east, 
"Tis neither good for man nor beast." 

Many indications and popular sayings are very 
familiar, some being common to all parts of the 
world. The far flight of birds, wild animals 



Descending Current of Air 127 

disporting themselves in the open air, and the 
exhilarating effects of fine bright weather upon 
human beings as well as the animal creation 
generally, are well - known accompaniments. 
The prevalence of morning and evening mists 
during hot weather, of fogs, white or black, as 
the case may be, and frost during winter (the 
necessary accompaniment of the absence of 
wind during anticyclonic conditions), as also 
the veering of the light breezes with the sun 
" in by day, and out by night," the land and sea 
breezes due to unequal heating of sea and land, 
experienced at maritime stations, are all more 
or less common phenomena accompanying this 
form of isobars. 

Whilst in cyclones it is considered that we 
have to deal with an ascending current, for anti- 
cyclones the current is a descending one, and, 
as stated, sometimes serves as a " feeder " to the 
former. The ascending damp air of the cyclone 
more or less nearly saturated with moisture 
favours precipitation and consequent rainy con- 
ditions, whilst the air being drier, and such 
moisture as is present far from the tempera- 
ture of saturation, in the anticyclone, it in 
general favours fine and settled conditions of 
weather. 

It has been already stated that both forms of 
circulation in general depend upon one another, 
and that cyclones frequently travel round anti- 
cyclones, near whose edges they have been 



128 Weather Science 

formed, though it sometimes happens that they 
will travel far from these latter, occasionally 
moving not only across the Atlantic into Europe, 
but even passing thence to Asia. 

In addition to the more common forms referred 
to, special groups of local disturbances of smaller 
size, variously known by the names of whirlwinds, 
tornadoes on land, and water-spouts on the sea, 
the simoon or dust cloud of the desert, the 
hurricanes of the West Indies, and the typhoons 
of the China Seas, are all related to cyclones, 
though "the motion of the wind in storms of 
the eddy type is probably more truly spiral, in- 
curving towards the centre, than circular " (Scott). 

The whirlwind or tornado may be described 
as a mass of air in rotation round an axis 
usually nearly vertical. It may be as much as 
200 feet in height, but its breadth does not 
usually exceed 10 feet. The harmless "dust 
whirl " of the roadside and the terrible tornado 
of America are extreme examples, whilst the 
"simoon" of the desert is a whirlwind carrying 
sand and dust. When the whirl is large and 
the air moist we sometimes get a thunderstorm, 
such as accompanies the "pampero" of the 
Argentine Republic, which is described as a 
south-west wind, ushered in by a sudden squall, 
with rain and thunder and a typical form of 
cloud wreath. In northern latitudes a similar 
kind of disturbance is known as a " line squall " 
(Abercromby). 



Tornadoes and Waterspouts 129 

The most characteristic feature of the tornado 
is its funnel or spout, the cylinder of air in 
rotation, whilst the system moves forward, 
usually in a north-east direction at a rate of 
about 80 miles per hour. The rotation is of a 
somewhat complex character ; in addition to the 
counter-clockwise or cyclonic motion there is 
usually a violent upward current, also a rising 
and falling motion, the end of the spout some- 
times rising from the ground and then descending 
again, whilst the axis is seldom upright, but sways 
slightly to and fro. Rain and thunder usually 
accompany this manifestation. Great damage 
is done by its destructive violence houses, 
trees, fences, churches, etc., being blown down 
or carried up into the air for a great distance. 
It has been estimated that the velocity of the 
wind rotating near the centre of a tornado may 
reach as much as 500 miles per hour; whilst 
the upward velocity may sometimes attain to 
over 100 miles per hour. The central column 
of rarefied air being cooled by expansion, any 
vapour within it is condensed. Thus some- 
times a "water-spout" is produced. That this 
water is not drawn up from the sea is shown 
by the fact that even when a water-spout passes 
over the sea, the water in it is quite fresh, and 
not at all salt. The funnel shape of the water- 
spout or tornado clouds is considered to be due 
to the increased pressure of the air near the 
surface. Above, the absence of friction and the 



130 Weather Science 

pressure from below causes the central rarefied 
area to extend somewhat, but lower down there 
is increased internal pressure, and the rotating 
air is confined to a narrow space. 

Tornadoes appear to be more common in the 
spring and early summer, whilst in autumn and 
winter they are of very rare occurrence. They 
usually occur on sultry days, and either in the 
south-east, or right front of cyclones, or in front 
of the trough of V depressions ( Abercromby). 

The destruction done by these fearful, though 
fortunately short-lasting manifestations, which 
"break the monotony of a tropical calm," has 
been so often described in glowing terms by 
different writers that it seems hardly necessary 
to repeat the well-worn tale. Those who are 
fond of complaining of the badness of our own 
weather may at least be reminded that we 
have much to be thankful for in our exemption 
from such catastrophes ; though besides this 
purely negative benefit, the existence of many 
positive advantages alluded to in the course of 
this work may with more justice cause our own 
climate (or at least that of the more maritime 
western regions), to be regarded as one of the 
best in the whole world, though perhaps that of 
certain stations in the Southern Hemisphere is 
more salubrious. 



Howard's Nomenclature 131 



CHAPTER VII 



CLOUDS HOWARD S NOMENCLATURE CIRRUS CIRRO-CUMULUS 

CUMULUS STRATUS CIRRO-STRATUS CUMULO-STRATUS NIMBUS 

HEIGHTS OF DIFFERENT KINDS OF CLOUDS MOTIONS OF CLOUDS AND 
AIR CURRENTS FOGS, MISTS, ETC. DUST PARTICLES OR OTHER 



IT has been often said that the study of clouds 
is one of the most important elements necessary 
for successful forecasting of coming weather. 
More than a century ago, Howard, in his essay 
on the " Modifications of Clouds," proposed the 
nomenclature for the different kinds which is 
now universally adopted. " Clouds," says he, 
"are subject to certain distinct modifications, 
produced by the general causes which effect all 
the variations of the atmosphere ; they are 
commonly as good visible indications of the 
operation of these causes, as is the countenance 
of the state of a person's mind or body." He 
first discriminates the three simple kinds, cirrus, 
cumulus, and stratus, the names being derived 
from the Latin, and nearly correspond to their 
general appearance. He defines cirrus cloud as 
" parallel, flexuous, or diverging fibres, extensible 
in all or on all directions " (nubes cirratd). The 
cumulus (nubes cumulata) cloud is a "convex 



132 Weather Science 

or conical heap increasing upwards from a 
horizontal base." The third form, stratus cloud 
(nubes strata), is " a widely extended, continuous, 
horizontal sheet, increasing from below." Of 
these three forms the cirrus clouds are the finest 
and most lofty ; cumulus, more dense and formed 
in lower regions of the atmosphere ; stratus, the 
lowest form, usually forming in the evening and 
dissipating towards the next morning. Howard 
distinguishes four intermediate forms, the cirro- 
cumulus, the cirro-stratus, the cumulo-stratus, 
and the cumulo-cirro-stratus or nimbus. Other 
forms are sometimes given, but these seven 
kinds will describe 90 per cent, of all skies 
(Abercromby). Howard's definition of these 
intermediate forms we take from his famous 
essay, mentioned above. The " cirro-cumulus " 
consists of" small, well-defined, roundish masses," 
in close horizontal arrangement. The "cirro- 
stratus," " horizontal or slightly inclined masses, 
attenuated towards a part or the whole of their 
circumference, bent downward, or undulated, 
separate, or in groups consisting of small clouds 
having these characters." Next comes the 
" cumulo-stratus," " the ' cirro-stratus ' blended 
with the cumulus and either appearing inter- 
mixed with the heaps of the latter or super- 
adding a widespread structure to its base. The 
seventh form, nimbus or cumulo - cirro - stratus, 
'the rain cloud.' A cloud, or system of clouds, 
from which rain is falling." " It is a horizontal 



Cirrus Clouds 133 

sheet above which the cirrus spreads, while the 
cumulus enters it laterally and from beneath." 

In addition to the above seven well-marked 
forms some writers distinguish between cirro- 
stratus and strata - cirrus, cirro - cumulus and 
cumulo- cirrus, according as one or other character 
predominates, Howard's forms cirro-stratus and 
cirro-cumulus being each subdivided into two. 
Captain Wilson Barker * proposes a very simple 
classification, considering all clouds as belonging 
to one or other of two types : (1) The Cumulus 
or heap type; (2) the Stratus or large sheet 
type, including all forms save Howard's cumulus 
under the second heading. Of all these forms 
the cirrus clouds are those which have the 
greatest elevation and variety of extent with 
least density. They are the earliest indications 
of change after a period of fair, settled weather, 
signs that this is to be followed by less favour- 
able conditions. 

At first there appears a few threads " pencilled, 
as it were, on the sky." These increase in length, 
and new ones are added laterally. Often these 
first threads serve as stems to support numerous 
branches, and from these in their turn other 
branches spring. 

Owing to their great height, cirrus clouds 
though often in fairly rapid motion, seem more 
slowly moving than clouds of other forms. It 
is fairly certain that they consist of small ice 

1 Essay on " Clouds and Weather/' 1895. 



134 Weather Science 

crystals, whence their effects in causing halos 
and other optical phenomena. Investigations by 
Hildebrandson in Sweden and the Rev. Clement 
Ley in this country have added considerably to 
our knowledge of atmospheric movements in the 
upper regions of the air. Cirrus clouds seem to 
be found in all parts of the earth, but those seen 
in tropical regions are probably at a greater 
altitude above its surface than the polar ones. 
With regard to their duration, Howard remarks 
that this varies from a few minutes to many 
hours, being long when they appear alone at 
great heights, and shorter when they are formed 
lower and in the vicinity of other clouds. In 
fair weather the sky is seldom quite free from 
small groups of oblique cirrus, whilst continued 
wet weather is attended by horizontal sheets, 
which subside quickly and pass to the cirro- 
stratus form. Before storms they appear lower 
and denser, and usually in the quarter opposite 
to that from which the storm arises. This form 
of cloud is often called "mare's tails," more 
especially when it is curved in form. In 
observing these clouds, it should be noted 
whether they are developed in any particular 
region of the sky rather than another, as well as 
the relation between their longitudinal extension 
and the direction in which they are moving. 

The cirro-cumulus is also a lofty cloud, though 
usually less high than the cirrus. It differs from 
the latter in being of a more rounded form, 




CUMULUS AND HIGH STRATUS. 



CIRRUS. 
(Photos by Capt. Wilson Barker, F.R.S.E., F.R.Met.Soc., &c.) 



To face p. 134. 




SQUALL CUMULUS. 




CUMULUS. 
(Photos by Capt. Wilson Barker, F.R.S.E., F.R.Met.Soc., &c.) 



To face p. 135, 



Cirro-Cumulus Cumulus 1 35 

consisting usually of small, detached masses, 
sometimes compared to a flock of sheep lying 
down, or the markings on the back of mackerel, 
whence the name " mackerel sky." It seems to 
be formed from a cirrus or from a number of 
small separate cirri, by the fibres collapsing and 
becoming small, roundish masses, the change 
taking place either through the whole mass at 
once, or gradually from one end to the other. 
This form is frequent in summer during warm, 
dry weather, and is more sparingly seen in 
intervals between showers, or before thunder- 
storms, when very dense and compact masses 
in close contact are often visible. 

The cumulus cloud, commonly called the 
" wool pack," is formed by an ascending current 
of air whose vapour is rapidly condensed. It is 
the densest kind of cloud formed in the lower 
atmosphere. Its lower surface is roughly plane, 
whilst its upper rises into conical or hemispherical 
heaps, which sometimes " continue nearly of the 
same bulk or rapidly rise to mountains." If 
remaining unchanged in size they are usually 
numerous and near together, when swelling they 
are few and far apart ; in either case their bases 
always lie nearly in one horizontal plane. These 
horizontal bases are, of course, evidence of the 
existence of strata of air of different temperatures. 
If the lower stratum be at a higher temperature 
than the upper it can contain more moisture, 
and thus will dissolve any portions of the cloud 



136 Weather Science 

which descend into it, so that the cumulus 
cloud appears to stand on the surface of separa- 
tion between these two layers of different 
temperature. In fair weather the variations of 
these clouds are often periodical during the 
course of the day. They begin to form some 
hours after sunrise, arrive at their maximum size 
during the afternoon, at the hottest time of 
the day, and then diminish, totally disappearing 
towards sunset. If, on the contrary, they increase 
rapidly in size, sink downwards, and do not dis- 
appear in the evening, rain may be expected. 
The formation of large cumuli to " leeward " in 
a strong wind indicates the approach of a calm 
with rain ; if they do not subside, but continue 
to rise towards sunset, thunder is to be expected 
in the night. In winter time the appearance of 
cumulus in the south after a fine day often 
indicates approaching snow. 

The stratus is a cloud lying in horizontal 
layers or strata, whence the name. Howard 
applied this term also to ground mists and fogs, 
but this is now discarded by meteorologists. It 
may be considered as the " cloud of night," since 
it owes its origin to the evening mists and 
grows denser during the night, dissipating again 
towards morning. The air being tolerably still 
and radiation from the ground going on, the 
general mass of the atmosphere above it cooling 
gradually, some stratum arrives at the dew point 
temperature and its moisture is condensed into 



Cirro-Stratus and Cumulo-Stratus 137 

cloud. The pure stratus cloud is an accompani- 
ment of fine weather, and if stratus at night be 
followed by diffuse fog in the morning we have 
generally settled atmospheric conditions. 

The cirro-stratus, according to Howard, appears 
to result from the subsidence of the fibres of the 
cirrus to a horizontal position ; at the same time 
they approach one another laterally. The form 
and relative position often suggests "shoals of 
fish." The structure is always thickest in the 
middle and thinner towards the edges. This 
form of cloud often precedes wind and rain, the 
nearer or more distant approach of which may 
sometimes be inferred from the greater or less 
permanence of these clouds. Owing to the 
great extent but little perpendicular depth of 
this form, the sun or moon may often be seen 
shining through it surrounded by a halo, so that 
the appearance of these phenomena is often 
regarded as a sign of approaching foul weather. 
It seems fairly certain also from these phenomena 
that the cirro-stratus clouds are largely composed 
of frozen particles of vapour. 

The cumulo-stratus cloud is a compound of 
cirro-stratus resting either on the top of a cumulus 
or crossing an isolated patch of the latter. It 
may be regarded as the cumulus cloud passing 
gradually into the nimbus form. It is usually 
a forerunner of rain or snow, according to the 
season of the year. 

The nimbus cloud is a name loosely given to 



138 Weather Science 

any kind of cloud from which rain falls, so that 
the term may be applied rather vaguely to 
different species. Abercromby distinguishes two 
kinds the cumulo-nimbus, the rocky, cumulus 
cloud from which rain falls in squalls or in 
showers; and pure nimbus, a flatter cloud more 
like heavy strato-cumulus, that forms from 
or under cirro - stratus. The name cumulo - 
cirro - stratus, suggesting its mode of origin, 
has been already alluded to. " The reason for 
making nimbus a class of its own comes from 
the fact that a sudden striking change comes 
over the look of the upper surface of a cloud 
the moment rain begins to fall " (Abercromby). 
This change is possibly associated with the 
discharge of electricity at the moment of 
precipitation. 

In addition to these principal varieties a 
number of minor forms, some of which are of 
importance in judging coming weather, may be 
distinguished. Sometimes before the approach of 
a cyclone a blue sky becomes white, then grey, 
and drizzling rain falls without the formation of 
any true cloud form. For this Mr Ley has 
given the name " cirrus haze " or " cirro-nebula." 
Small detached clouds seen in rapid motion 
under any mass of cloud just before the pre- 
cipitation of rain are frequently called " scud," 
or sometimes fracto-cumulus. Before the advent 
of squalls and thunderstorms there is sometimes 
seen a long roll of narrow black cloud in rapid 



Height and Motion of Clouds 139 

motion, and this form goes by the name of 
" cloud wreaths." 

With regard to the height above the surface 
of the ground at which the various cloud forms 
exist, we may class cirrus, cirro - cumulus, and 
cirro-stratus as high clouds, since they often exist 
at an altitude of 20,000 to 30,000 feet ; cumulo- 
and strato-cirrus forms are found at intermediate 
altitudes, whilst cumulus, stratus, and nimbus are 
low, many being below 2,500 feet. The levels 
for Upsala of the principal varieties in summer 
have been given as: cirrus forms, 20,000 to 
27,000 feet ; middle forms, 12,000 to 15,000 feet ; 
cumulus, nimbus, and stratus, below 6,000 feet. 
Though these altitudes vary with latitude and 
season of the year they serve to illustrate the 
principle, " that clouds everywhere tend to form 
at a few definite levels, widely separated from 
each other." 

Motion of clouds. Low-lying clouds usually 
move in the same direction as that of the wind 
felt at the surface of the ground, though in 
mountainous regions they are subject to local 
variations. Their motion is, however, always 
more rapid than that of the air current close to 
the ground. Clouds at great altitudes have been 
carefully studied, especially in recent years, by 
the observers at Blue Hill, near Boston, U.S.A., 
and elsewhere, and it has been shown that at 
a height of about 5 miles the movement is 
practically three times as fast in summer and 



140 Weather Science 

six times as fast in winter as the currents at the 
earth's surface. 1 As a general rule, the greater 
the altitude the faster the movement. Their 
motion, too, is often in a different direction from 
that of the surface currents, and sometimes we 
find several layers of clouds floating at different 
heights, each moving in different directions. 

Fogs and mists are closely related to clouds. 
Aqueous vapour rising by evaporation from the 
ground is itself invisible, but it becomes con- 
densed in the form of minute droplets, with the 
liberation of its latent heat. At any definite tem- 
perature air has the power of maintaining a cer- 
tain quantity of aqueous vapour in the gaseous 
condition, this being greater as the temperature 
is higher. When the air contains the maximum 
quantity of vapour for a given temperature it is 
said to be saturated, and the smallest diminution 
of temperature causes a portion of the vapour 
to leave the gaseous condition and become 
"precipitated," as it were. Two masses of air 
of different temperatures mixing together, each 
saturated with moisture, producing a mixture of 
an intermediate temperature, a portion of their 
moisture is thus condensed and a fog is formed. 
The condensed particles having a tendency to 
form on solid matter floating in the air, such 
as dust and soot, etc., we get the black fogs of 
London and other large towns. No fogs can 

1 Inwards, presidential address to Royal Meteorological Society 
' ' On Some Phenomena of the Upper Air." 




CIRRO CUMULUS. 



\ 



GROUND FOG. 
(Photos by Capt. Wilson Barker, F.R.S.E., F.R.Met.Soc., &c.) 

To face p. 140. 



Fogs and Mists 141 

be produced in an atmosphere perfectly free 
from foreign particles by the condensation of 
aqueous vapour alone ; this has been shown by 
Aitken and Tyndall. In addition to the cooling 
produced by the mixture of two masses of air of 
different temperatures, fogs may be produced by 
the passage of a warm damp current over a cold 
surface, and also by the passage of saturated air 
over a warm water surface. The water, being 
warmer than the air, gives off more vapour than 
the latter can contain, and thus a fog is pro- 
duced. Of course no fog can be formed in 
windy weather, since it is dissipated by the least 
motion of the air. Mists are similar in character 
to fogs, but the particles are larger, and they feel 
wetter. They are more commonly found on 
parts of hills covered with trees, and near the 
banks of rivers and marshy places, than elsewhere. 
A cloud is in reality only an extensive fog or 
mist existing at a greater altitude, and its forma- 
tion, as at a mountain top, is due to the con- 
densation of moisture from the warm lower air 
in passing over it. This has its temperature 
lowered and is thus forced to deposit part of 
its contained moisture. 

In making observations of meteorological 
phenomena it is customary to estimate the 
amount of cloud visible in the sky at the time 
of observation. The ordinary estimation is by 
tenths, being recorded for a clear blue sky, 
10 for one altogether covered by cloud. Varia- 



142 Weather Science 

tions in estimation as to the amount of cloudi- 
ness naturally prevail to a considerable extent, 
and the same cloud appears of a very different 
size near the horizon than when higher up in 
the sky. With regard to the speed of clouds 
generally much information has been recently 
gained. The Blue Hill (U.S.A.) Observatory 
results give an average speed of about 20 miles 
per hour for stratus clouds at an altitude of 
about 2,000 feet, whilst for cirro-cumulus at 
4 miles high the speed increases to 70 miles per 
hour, and for very lofty cirrus (5 to 6 miles above 
the ground) this may amount to as much as 
80 miles per hour. It has been asserted that 
speeds of 250 miles per hour have been observed ! 
(Archibald). For every 1,000 feet of ascent add 
on about 2 miles an hour to the velocity of 
motion (Archibald). The speeds are greater in 
winter, but the average height of clouds are 
greater in summer than in winter. The observa- 
tions at Upsala and other places in Europe 
give less rapid but still large speeds for the 
upper clouds. Speeds of 19 miles per second 
for clouds at 4,300 feet, and 38 miles per hour 
for clouds at 38,000 feet were noticed. These 
rapid movements of the upper air may eventually 
be utilised in " flying machines " travelling with 
the wind. The successive cloud layers, which, 
as we have already said, tend to form at about 
certain definite levels, coincide with air streams 
differing from one another in point of velocity, 



Formation of Clouds 143 

temperature, and humidity, and these must 
exercise a marked influence on the weather con- 
ditions below. 

We shall next devote a few lines to the 
question of the formation of clouds. Every 
cloud is the visible top of a column of invisible 
water vapour, sometimes stretching from the 
ground upwards, and becoming condensed on 
reaching a colder stratum of air. At every 
temperature a certain proportion of water vapour 
may be maintained in the gaseous state, but 
when this amount is exceeded, or the air is 
"saturated," the excess is usually condensed, 
though it has been found that perfectly clear 
dust-free air may be supersaturated ; but when 
fine particles of solid matter are present con- 
densation takes place round these as nuclei. 
It has been supposed that sudden rainfalls are 
due to this condition of vapour saturation, but 
Mr Aitken has found that the presence of dust 
is absolutely necessary to the formation of rain. 
The amount of water in the air rapidly decreases 
as we go upwards, though cirrus clouds at a 
height of 50,000 feet have been occasionally 
noticed, these latter being probably composed of 
small ice particles or "needles." The shallow 
stratus, cirro - cumulus, and cirro - stratus clouds 
are supposed to be due to the mixture of a 
layer of warm air with an underlying colder 
stratum. When one current crosses another 
it raises waves in the latter; the "mackerel 



144 Weather Science 

sky," and the long rolls of dark cloud following 
one another at the rear of a storm, with 
showers and brighter intervals, being examples 
of such aerial waves. The clouds around 
mountain-tops are due to the cooling produced 
by these latter causing condensation of part 
of the moisture in the air which is rising 
upwards. 

The "table-cloth" over Table Mountain and 
other hills is formed by the passage of a warm 
moist current of air over the cold hilltop, whose 
action condenses part of this moisture. When 
it passes beyond the mountain the cloud mixes 
with warmer air, and is once more dissipated, 
but as fresh air is continually rising, a cloud 
is almost constantly formed over the mountain, 
but consists of constantly changing particles of 
water. The clouds occurring in connection 
with cyclones are due to the ascent of damp 
air rising and mixing with the drier atmosphere 
above. 

The literature of cloud prognostics is very 
extensive, and from all ages the portents and 
signs of coming weather changes have been 
discussed with more or less ingenuity by many 
writers. The remarks of the late Admiral 
Fitzroy, though given elsewhere, may be here 
quoted : 

" After fine weather the first signs in the sky 
of a coming change are usually light streaks, 



Cloud Prognostics 145 

curls, wisps, or mottled patches of white distant 
clouds, which increase, and are followed by an 
overcasting of murky vapour that grows into 
cloudiness. The appearance, more or less oily 
or watery, as wind or rain may prevail, is an 
infallible sign. Usually the higher and more 
distant such clouds seem to be, the more gradual 
but general the coming change of weather will 
prove." 



Further remarks by the same author with 
reference to the motions, colours, etc., of cloud 
forms, are quoted in Mr Inward's valuable 
" Weather Lore." Some of the descriptions of 
these phenomena to be found in the Bible show 
the results of careful observation on the part 
of the scriptural writers. Many references to 
cloud phenomena are especially to be found in 
the book of Job. In Aristophanes' comedy the 
" Clouds," written as a satire on Socrates and 
his teaching, "the clouds" are supposed to be 
the new, hitherto unknown deities introduced 
by that philosopher to the Grecian world, to 
replace the gods and goddesses of Olympus. 
Shakespeare's description of some cloud appear- 
ances and changes may be worth quoting 
here : 



" Sometimes we see a cloud that's dragonish 
A vapour sometimes like a bear or lion, 
A towered citadel, a pendant rock, 
A forked mountain, a blue promontory 



146 Weather Science 

With trees upon't that nod unto the world 
And mock our eyes with air. 
That which is now a horse, even with a thought 
The rack dislimns and makes it indistinct 
As water is in water." 

Of a coming storm he says in the Tempest : 

" And another storm brewing ; 
I hear it sing i' the wind 
Yond' same black cloud, 
Yond' huge one, 
Looks like a foul lumbard 
That would shed his liquor . . . 
Yond' same cloud cannot chuse 
But fall by pailfuls." 

The shepherd of Banbury is full of weather 
wisdom from the appearances of clouds. When 
they increase, we are told: "If the sky from 
being clear becomes fretted or spotted all over 
with bunches of clouds, rain will soon fall." 
Again, with regard to wind, he says : " If you 
see a cloud rise against the wind, when that 
cloud comes up the wind will blow the same 
way," etc. As signs of a coming storm : " In 
summer, when wind has been south for two 
or three days, and it grows very hot, you see 
clouds rise with great white tops like towers, 
there will be thunder and rain suddenly. If 
two such clouds arise, one on either hand, it 
is time to make haste to shelter." The appear- 
ance of cloud hats or "caps" on distant hills, 
especially when these lie to the south or south- 
west of the observer, is looked upon as a sure 



Cloud "Hats" 147 

sign of approaching rain. Many such sayings 
are common in Scotland 

" When Largo Law puts on his hat. 
Let Kellie Law beware of that ; 
When Kellie Law gets on his cap, 
Largo Law may laugh at that," 

the latter being to the south - west of the 
former. 



CHAPTER VIII 



THE WINDS DIRECTION COMPASS AND TRUE BEARINGS RELATION 

TO ISOBARS BEAUFORT'S SCALE BUY BALLOT'S LAW TRADE 

WINDS GENERAL CIRCULATION SPECIAL WINDS I FOHN, SCIROCCO, 

BORA,, MISTRAL, LAND AND SEA RREEZES NOTE ON WIND 

VELOCITIES AND BEAUFORT'S SCALE. 



THE wind is a body of air in motion, and this 
motion being produced by differences of baro- 
metric pressure in different directions, it is found 
that as a general rule the "force" or velocity 
of its movement is roughly proportional to the 
closeness of the isobaric curves, being consider- 
able when they are near together, and small 
when they are wide apart. The direction of 
the wind is recorded according to the point of 
the compass from which it blows, and it is per- 
haps advisable in this connection to recollect 
that the north - seeking end of the magnetic 
needle does not point exactly towards the true 
or geographical north, but at present (1911) 
makes an angle of about 15 to the west with it, 
for the neighbourhood of London. This " varia- 
tion " is slowly changing from year to year, at 

148 



Wind Directions 149 

present diminishing, and in about fifty years' 
time true and magnetic north will coincide. 
It is customary to divide the whole circum- 
ference of the compass card into thirty-two parts 
called points, each point making thus an angle 
of 360 -4- 32 = 11 J, with the neighbouring points 
on either side of it. In practice it is usually 
sufficient in estimating the direction of the 
wind to use eight principal points only, N., 
N.E., E., S.E., S., S.W., W. and N.W. In 
estimating the closeness of any two isobars, 
on which the force of the wind depends, we 
take the slope of the barometric gradient, 
measured at right angles to the isobar curves, 
this being the shortest line which can be 
drawn between them. Gradients are measured 
by the number of millimetres of barometric 
pressure difference in one geographical degree, 
or their equivalents in English measure, 1 
mm. = 0*04 inch, and 60 nautical miles = length 
of 1. According to its intensity the wind is 
variously designed as light, moderate, fresh, 
strong, gale, storm, or hurricane. The well- 
known Beaufort's scale is still in very general 
use. On this system an arbitrary series of 
numbers, from to 12, "calm" to "hurricane" 
is taken to estimate approximately the various 
intensities of wind corresponding to velocities 
from to 100 miles per hour. 

The following numbers give the approxi- 
mately equivalent velocities of the wind as 



Weather Science 



determined by the British Meteorological 
Office : 



Velocity in miles 
Wind. per hour. 

7 Moderate gale . 40 

8 Fresh . 48 

9 Strong . 56 

10 Whole . 65 

11 Storm . . 75 

12 Hurricane . 90 

and upwards. 



V( 
Wind. 

Calm 
1 Light air . 
2 Slight breeze 
3 Gentle 
4 Moderate 
5 Fresh 
6 Strong 


slocity HI miles 
per hour. 

. 3 
. 8 
. 13 
. 18 
. 22 
. 28 
. 34 



[See also note at end.] 



Though, as we have said, the force of the wind 
varies directly with the closeness of the baro- 
metric gradients, yet for any given gradient in 
this country, winds from north and east are 
stronger than those from the south and west 
points, by at least one-third of their whole 
amount. In certain cases of tropical and sub- 
tropical winds, such as the " northers " of New 
Mexico and the " nortes " of Panama, the force of 
the wind is quite disproportionate to the gradient. 
There are also some few winds distinctively 
called "non isobaric," whose origin does not 
appear to be due to differences of barometric 
pressure. Since in general, whenever there is a 
difference of pressure, the air must flow from a 
region of high pressure towards one where it is 
low, we see at once that there must be a relation 
between wind and air pressure. The law express- 
ing this relation is known by the name of Buy 
Ballot, a Dutch professor who drew attention to 



Buy Ballot's Law 151 

its importance. It is commonly enunciated in 
the following terms: 

" If you stand with your back to the wind in 
the Northern Hemisphere, the barometer will be 
lower on your left hand than on your right. In 
the Southern Hemisphere, standing with your 
back to the wind, you will have a lower baro- 
meter on your right hand than on your left." 

Thus (in our latitudes) if the barometer is 
higher to the north than to the south, the 
wind will be east, southerly if the pressure is 
higher to the east than the west, and so on. 
Thus in every case in the Northern Hemisphere 
" whenever we find an area of low readings, the 
wind moves round it against watch hands, and 
whenever we find an area of high readings, the 
wind moves round it with watch hands " (Scott). 
The converse is the case for the Southern 
Hemisphere. 

Though in our own country we are accustomed 
to regard the direction of the wind as a symbol 
for all that is most variable, yet in other regions 
there is more regularity in the phenomena of 
its motion. Two main currents may be distin- 
guished, the equatorial warm current northwards 
and southwards towards either Pole, and the 
polar cold current from either Pole towards the 
Equator. The cause of these great atmospheric 
currents is of course the difference of temperature 
between the equatorial and polar regions. Air 



152 Weather Science 

heated in the former regions, becoming lighter, 
ascends, and its place is taken by colder air flow- 
ing in from other parts. Hadley, nearly two 
hundred years ago, pointed out that a mass of 
air moving polewards will be deviated towards 
the east by the effect of the earth's rotation, 
since it is coming from a region of quicker 
rotation towards one of slower movement, and 
conversely the polar currents will be deviated 
in the opposite direction (or lag behind). His 
theory was, however, imperfect in that he 
assumed that only currents moving along the 
meridian (i.e., due north or due south) are thus 
affected, but it was shown by Poisson in 1837, 
that the effect of the earth's rotation on a freely 
moving mass near its surface, is to cause a devia- 
tion to the right in the Northern Hemisphere, and 
to the left in the Southern Hemisphere, inde- 
pendently of the direction in which the mass 
may be moving. Ferrel during 1858 and 1859, 
developed a theory of atmospheric motions, 
based on this theorem of Poisson, which he 
independently discovered. He gave briefly the 
theory of cyclones, tornadoes, etc., and showed 
why the law of Buy Ballot is true. 

We have said that air currents from the 
Equator into higher latitudes are deflected by the 
earth's rotation, and thus in the Northern Hemi- 
sphere the south wind becomes a south-west one, 
and the polar current flowing equatorwards be- 






Trade Wind Zones 153 

comes a north-east one. The latter is known as 
the north-east trade wind, and blows with great 
persistency over the Northern Atlantic and 
North Pacific Oceans. In the Southern Hemi- 
sphere we have the south-east trade winds. Near 
the Equator, we have a calm belt occasionally 
broken by violent storms. The trade-wind zones 
as well as the intervening belt of calm, shift 
their position somewhat during the course of 
the year, being about 10 lower in March than 
in September. Thus the north-east trade wind 
blows between latitudes 25 and 3 N. in the 
spring, but in September its position is between 
about 35 N. and 10 N. The "calm" and 
" south-east " trade zones undergo corresponding 
changes. In Southern Asia and over the Indian 
Ocean, we find the well-known "monsoons" 
blowing for one half the year in one direction, 
and for the other half in the opposite direction. 
The south-west monsoon blows between May 
and October over the Northern Indian Ocean, 
whilst the north-east trade wind blows during 
the rest of the year. 

South of the Equator the south-east trade 
blows from May to October, and the north- 
west monsoon "formed from the north-east 
trade wind drawn across the Equator," blows 
from October to May. 

The following is in a few words the most 
general account of the circulation of the air, and 



154 Weather Science 

variation in mean barometric pressure through- 
out the globe. We have first a zone over the 
Equator at which the pressure is about 29*8 inches 
(of mercury), on either side of which there is 
a belt of higher pressure (about latitude 30 c 
N. and S.), reaching to 30'2 in. Within this 
area the trade winds blow throughout the year, 
except over the north part of the Indian Ocean, 
where in July they blow inwards towards an area 
of low pressure and high temperature, the " south- 
west monsoons," whilst in January, the " north- 
easterly " winds prevail over this region. Further 
north and south (polewards) of these regions of 
higher pressure, the winds in general blow 
towards the poles. 

Throughout Europe the most frequent wind 
is the south-western, whilst in Asia and Eastern 
North America the north-west wind is perhaps 
more prevalent. On the whole, from Hann's 
investigations, it may be stated that the warmest 
winds, the southerly and westerly, produce a 
mean elevation of temperature in Central Europe 
of from 2 to 6 above the value it would other- 
wise have, whilst on the other hand, the northerly 
and easterly winds produce a lowering of from 
5 to 7, the north-east causing the greater 
depression. The conditions are otherwise in 
Asia, the most frequent wind, the north-west, 
lowering the temperature as much as 4*5, 
though on the other hand, the south wind 
raises it by 10*4, but is a very rare phenomenon 



Wind Variation 155 

(Scott). This predominance of the south-westerly 
winds over Europe is not a general phenomenon, 
but, speaking generally, " in winter the air flows 
off the land on to the sea, and in summer it flows 
off the sea on to the land " (Scott). The latter 
are the " rain bringers," the former usually dry 
winds. Thus in our country we find the south 
and south-west winds are the rainy ones (and 
warm), the north-east wind is cold and dry. 
The late James Glaishers, F.R.S., for many 
years in charge of the meteorological work 
of Greenwich Observatory, gave the following 
as the average number of hours per annum 
that the wind was in each one of the eight 
principal " points," from the mean of ten years' 
observations. 

The wind was N. on 827'2 hours. 

N.E. 1018-9 

E. 599-4 

S.E. 566-4 

S. 641-0 

S.W. 2737-4 

W. 1252-7 

N.W. 557-8 

Calm 564-0 

[Journal of the Royal Meteorological Society, vol. i.] 

Of various local varieties of wind distinguished 
by special names, we may refer to the Fohn 
wind of Switzerland, the "North- Westers" of 
New Zealand and similar hot currents, the 
"land and sea" breezes so common in hot 
countries, but not altogether unknown in our 



156 Weather Science 

own, the cold " blizzards " and " barbers " of 
North America, etc. The Fohn wind is a hot 
current of air which, at first moist, expands in 
passing upwards on the sides of the mountains, 
precipitates most of its vapour and becomes 
compressed and hotter in descending on the 
opposite sides. The Chinook wind of Canada is 
of a similar nature, and blowing warm from the 
Rocky Mountains, helps to raise the temperature 
of the plains below. In North Greenland a 
warm south-east wind sometimes blows from 
the interior mountain regions. A similar origin 
is to be looked for in the case of the " North- 
Westers" of New Zealand, the hot winds of 
South Africa, and of parts of Australia. The 
dry, parching air of the "Nor* - wester " after 
a while gives way to the " Southerly buster," 
bringing coolness and rain in abundance. The 
" Scirocco " or south - easterly wind of Italy 
and Sicily has also, perhaps, the character of 
a dry, hot wind, but there are contradictory 
accounts of its nature ; relief from its scorching 
action comes with the advent of the northerly 
" Tramontane" The " Harmattan " of West 
Africa is a hot east wind, blowing from the 
desert, and bringing with it clouds of red dust 
far out into the Atlantic. A similar wind from 
the desert blowing towards Spain is known in 
that country as the "leveche," and in Egypt 
the "khamsin," from the Arabic (for fifty) since 
it is considered to blow for fifty days. 



Cold Winds 157 

In North America cold, often snow-bearing 
winds are sometimes designated by the name 
of "Barbers" or "Blizzards," and the latter 
name has been applied by the newspaper press 
to any cold wind during the winter, accompanied 
by driving rain, snow, or sleet. The " Mistral " 
of the South of France, the "Bora" of the 
Adriatic, and the " Nortes " of Mexico are cold 
northerly winds, coming at the rear of cyclonic 
disturbances. The Bora acquires its specially 
cold, penetrating feeling, coming down from a 
lofty plateau to lower regions ; the Mistral, 
formerly considered to arise from a sudden 
cooling of the wind passing over the Pyrenees 
or the Alps, was shown by Marie Davy and 
Kaemtz to be due to more remote causes. 
Whenever it blows in Provence there is a 
region of high pressure to the east of the Gulf 
of Lions (Flammarion), and the violence of the 
wind is due to the shape of the "Pyrenean 
isthmus." 

f 

" Since the general direction of the atmos- 
pheric movement extends slightly towards 
the north - west, the central plateau of the 
Alps bends the air currents towards the Gulf 
of Lions. Cooped up between the Alps, the 
Pyrenees, and the Cevennes, it becomes a kind 
of ' rapid ' over Languedoc, thence arises the 
high pressure to the north-west of the Cevennes 
and low pressure over the Mediterranean. From 
this cause the violence of the north wind in 



158 Weather Science 

the Rhone valley. The mistral is the driest 
wind in these regions, because it becomes dried 
in its passages over the Cevennes, for it is rainy 
on the north-western slopes." FLAMMARION. 

This wind is the exact opposite to the Fohn. 
An old couplet says 

" Le Parlement, le Mistral et la Durance, 
Sont les trois fleaux de la Provence," 

thus classing it as one of the three plagues 
of the land of Troubadours and Trouv&res. 

A few words on the local winds known as 
land and sea breezes may fitly conclude this 
chapter. Most commonly towards noon on 
coast regions a breeze sets in blowing from the 
sea towards the land and gradually decreases in 
the afternoon, dying away at sunset. Towards 
midnight the air begins to blow in the opposite 
direction, from the land towards the sea. This 
action decreases after a while, and by sunrise 
the air is calm again. The old explanation of 
this circulation was that the land by day being 
hotter than the sea gives rise to ascending 
currents which are replaced by cooler air from 
the sea, whilst at night the land being cooler 
than the sea, the air immediately over it, colder 
and denser, flows outwards till the pressure over 
sea and land is equalised. This is still given in 
some text books, but Laughton, Blandford, and 



Land and Sea Breezes 159 

others have doubted the validity of this explana- 
tion. Mr Blandford considers that when the air 
over the land is expanded by heat and raised, the 
upper strata move off towards the cooler sea, 
and produce increased pressure at some distance 
from the land. The air flows from this high 
pressure region towards those positions where 
the pressure is less, and so we get a sea breeze 
" setting in from the offing," as Dampier long ago 
pointed out, not a wind drawn in by suction. By 
night the action is reversed, and the atmosphere 
over the land is cooled and contracts ; an isobaric 
slope is thus created, and the upper air slides 
down from the sea, sinking over the land and 
pushing out as the land breeze (Scott). The 
surface land breeze of evening is often accom- 
panied by squalls of considerable strength. As 
was to be expected, the sea breeze is generally 
moist, the land breeze dry, and sometimes 
deleterious in its effects. 

Such breezes are light on a low flat island or 
coast, but when there is a range of mountains 
not far from the sea, these winds are sometimes 
much stronger. The sea breeze of Jamaica 
owing to the proximity of the Blue " Mountains," 
is often very powerful. 

Note on Wind Velocities. The velocities corresponding to the 
numbers of the Beaufort scale, are sometimes calculated by an 
approximate formula, V = 1'87N/B 3 , or by the simpler V + 3=5B, 
where V is the wind velocity in miles per hour, B, the numberi 



160 Weather Science 



1, 2, 3, etc., of the Beaufort scale. Thus from the second formula 
we have the corresponding numbers, 

Nos. Beaufort scale 12345 6 

Wind velocity 

(V+3=5B) 2 7 12 17 22 27 

From the first formula 

(V=l-87x/B 3 ) 2 5 10 15 21 28 

Old values (0=3) (1 = 8) (2 = 13) (3 = 18) (4=23) (5 = 28) 

the numbers agreeing sufficiently well calculated by either form, 
but (1) on this scale corresponds more closely to (0) on the older 
form (2) to (1), etc., as will be seen by the figures in brackets. 
(See also the introduction by Dr W. N. Shaw, to a recent publica- 
tion of the Meteorological Office by Commander Hepworth, on 
"Trade Winds in the Atlantic," etc., 1910). 



Composition of Atmosphere 161 



CHAPTER IX 

MISCELLANEOUS PHENOMENA SNOW HAIL DOVE'S THEORY VOLTA's 

THEORY DEW FOG MIST RAINBOWS HALOS (t CORONJE " 

THE MIRAGE BLUE OF THE SKY TINTS OF SUNRISE AND SUNSET 

DUST. 

OUR earth, seven - tenths of whose surface is 
covered by water, is surrounded by an envelope 
of air known as the atmosphere, which presses 
on all bodies at the level of its surface with a 
pressure of nearly 15 Ibs. per square inch, and 
extends to an unknown height above. Its 
density is greatest at the earth's surface, and 
rapidly decreases as we go upwards, but no 
certain limits can be assigned beyond which 
we can positively assert that there is no atmos- 
phere at all, though at a height not greatly ex- 
ceeding 100 miles from the ground the quantity 
of air must be very small indeed. This atmos- 
phere is composed of a mixture of several gases 
oxygen, nitrogen, argon, and carbon dioxide 
with a variable quantity of water vapour, small 
traces of nitric acid, ammonia, etc., and minute 
quantities of almost all other substances. Were 
the whole atmosphere of uniform density, its 
height would be only about 5 miles, and this 



1 62 Weather Science 

height (26,000 feet) is sometimes called the height 
of the homogeneous atmosphere, and would pro- 
duce a pressure at the earth's surface equal to 
the actually existing pressure. This air pressure 
on every square inch is equal to that produced 
by the weight of a column of mercury 30 inches 
high and 1 square inch cross section (mean height 
of the barometer). Stated in terms of the 
C.G.S. system (in which the unit of length is 
1 centimetre, the unit of mass is the gramme, 
and the second the unit of time) the air pressure 
on each square centimetre is equal to that of 
a column of mercury 76 cm. high and 1 sq. cm. 
in cross - section. This pressure is known as 
1 atmosphere of pressure, and expressed in English 
measure it is equal to 14*7 Ibs. per square inch. 
The pressure becomes less and less the higher 
we go up. Thus Pascal found that the pressure 
diminished in ascending the Puy de Dome by 
about 7 inches (of mercury), from 30 inches at 
the bottom to 26 inches at the top (see also 
Introductory chapter). 

The amount of aqueous vapour contained in 
the air varies considerably from time to time. 
At any given temperature, however, there is a 
maximum amount which can remain in the 
gaseous condition, and when the air contains 
this amount it is said to be saturated. The 
amount necessary for saturation varies with the 
temperature, being greater as the temperature is 
higher and less when the temperature is lower. A 



Saturated Air 163 

rise of temperature in a quantity of saturated air 
thus enables it to maintain an increased quantity 
! of water vapour within it, whilst a fall of 
I temperature causes part of its moisture to be 
i deposited in the form of rain (or dew). A 
mixture of two masses of air at different 
temperatures, more especially if the warmer air 
is saturated, will in general cause a condensa- 
tion of some part of the contained moisture, 
though when saturated cool air is mixed with 
unsaturated warmer air, condensation need not 
occur. This is illustrated by the well-known 
example : " If we open the door of a laundry 
on a cold day a fog instantly forms. If we 
open the door of an ice-house on a warm day, 
no fog appears." The former is a case of the 
mixture of a saturated mass of warm air with 
colder unsaturated air ; the latter is a mixture of 
saturated cold air with warmer unsaturated air. 
Air coming from regions whence it has derived 
much moisture, as winds blowing in from the 
sea and rising in currents over the tops of hills, 
etc., is at first saturated, and being chilled by 
expansion and contact with the colder land 
surfaces, deposits most of its moisture in the 
form of rain. Thus the south-westerly and 
westerly winds, arriving first on our western 
coasts, come nearly saturated with moisture, 
which they have acquired by blowing over 
extensive surfaces of ocean ; coming to the hilly 
western counties they lose much of this moisture, 



164 Weather Science 

hence the heavy rainfall of those regions, though 
sufficient remains to make the south-west wind 
a rainy one everywhere in Britain. 

Snow is merely the solid form of rain, which 
takes the place of the latter whenever the 
temperature of the surrounding air is at or 
below the freezing-point of water. It differs 
from ice in being opaque instead of transparent ; 
this is due to the entanglement of air bubbles, 
etc. It is often considered that the formation 
of snow takes place directly from the cooled 
vapour without passing through an intermediate 
liquid condition. Snow falls in flakes, whose 
size varies considerably, and these flakes are 
found to be made up of crystalline figures of 
definite symmetrical shapes, having the general 
" hexagonal " character, six- pointed stars, snow 
flowers (frequently with a well-marked centre), 
or hexagonal plates. Whatever the variety of 
their forms, the angles made by adjacent faces 
or by the rays of the "stars" have a definite 
relation to one another, being either 60 or small 
multiples or sub-multiples of that angle. Being 
much less dense than rain a fall of snow occupies 
much greater space than an equivalent amount 
of rainfall ; as a rough rule it has been said that 
one foot of snow is equivalent to one inch of 
rain, but snow is more conveniently measured 
by thawing whatever collects in the rain gauge, 
either by adding a measured volume of hot 
water, or by directly warming and melting the 



Theories of Hail 165 

snow itself (a much slower process). Snow, 
though solid, evaporates slowly, and may thus 
disappear on a fine sunshiny day without thaw- 
ing, just as it is supposed to be formed with- 
out passing through the intermediate liquid 
condition. 

Hail is allied to snow in character, being 
also "frozen rain," but is denser and more 
compact. It is customary to distinguish two 
kinds : soft hail or " graupel," and true hail 
(Scott). It is probably due to the freezing of 
raindrops in their passage through strata of air 
colder than those in which they were formed. 
Dove's theory was that hailstorms are whirl- 
winds, whose axis is rather horizontal than 
vertical, as is commonly the case with other 
forms. The growing hailstones are swept round 
and round from hot to colder strata of air 
alternately, water collects on them in the warm 
strata and is frozen in the colder. After a 
time, by their weight, they fall to the ground. 
Volta, from the fact that hail often falls during 
a thunderstorm, and is associated with disturb- 
ances of the electrical equilibrium, propounded 
a singular theory. He supposed that two layers 
of cloud, one above the other, charged with 
opposite kinds of electrification, kept the hail- 
stones continually moving up and down by 
alternate attraction and repulsion, and devised 
an experiment called " electric hail " to illustrate 
this, Two metal plates placed one over the 



1 66 Weather Science 

other are taken, the upper one is connected 
with the conductor of an electrical (frictional) 
machine, the lower one put to " earth." If a 
number of pith balls are placed on the under 
plate, when the machine is set working, the 
balls fly rapidly to and fro between the plates. 

Of the two kinds of hail, soft hail is composed 
of small, rounded, soft pellets and commonly falls 
in dry weather. This kind is called " graupel " 
in Germany. The other kind, or true hail, is 
composed of larger, irregular, more or less con- 
centric layers of hard and soft ice, occasionally 
showing traces of crystalline structure, whose 
form resembles that of crystals of calc-spar. 

Very large hailstones have been known to 
fall from time to time. It is recorded that in 
1697 Robert Taylor, in Hertfordshire, found 
hailstones 14 inches in circumference. In tropical 
countries, and even in certain parts of Europe, 
much damage is done to vineyards, standing 
crops, etc., by their impact, and hail insurance 
is a care for the prudent farmer who does not 
wish to have the fruits of his labour wasted by 
a storm. Flammarion relates that the hail- 
storm of 13th July 1788 which passed over 
Northern France ravaged 1,039 parishes, and 
did damage to an amount estimated at 24,690,000 
francs (990,000 nearly). The largest hailstones 
weighed 250 grammes (9 ozs.), but it has been 
asserted that at times masses weighing two or 
three times as much have been picked up. 



Wells' Theory of Dew 167 

Stories of stones weighing pounds, or even as 
large as an elephant, may be disregarded. 

Dew is a phenomenon whose appearance 
seemed very mysterious in early days, and 
fanciful theories as to its cause were held 
without any serious grounds. Its explanation 
was very beautifully given by the late Dr Wells, 
whose well-known essay is often instanced as a 
model of the " inductive method." The air, 
as we know, always contains more or less 
moisture in suspension in the state of gas, the 
possible amount that can be contained varying 
with its temperature, being greater at higher 
than at lower temperatures. The method by 
which we " dry " things, i.e., cause their moisture 
to be evaporated into the surrounding air, depends 
upon this, we warm them. At any temperature 
the air can contain so much moisture and no 
more, and when this maximum amount is 
present, it is said to be saturated, and at the 
temperature of the " dew point." A fall of 
temperature, as by contact with colder bodies, 
then causes some of this moisture to be deposited 
in the form of drops of "dew," since the air, 
containing its full amount of moisture for the 
higher temperature, which exceeds the possible 
amount at the lower, part of this moisture 
must now take the liquid form, since at every 
temperature a definite proportion of vapour 
only can be retained. On a fine sunny day 
evaporation goes on, and the air takes up much 



1 68 Weather Science 

moisture. During the evening the temperature 
falls, more heat being radiated from the ground 
than is received by the latter from the now 
declining sun. If the sky be clear, this radiated 
heat rapidly passes out into space, but if, on 
the other hand, the sky be cloudy, then much 
of the heat will be reflected back again. In 
the former case the ground temperature will 
fall quickly, in the latter more slowly. The 
air in contact with the ground will fall to the 
same temperature as the latter, and if it contain 
much moisture it will soon reach the dew point, 
after which part of its vapour will condense. 
This will be deposited first on those substances 
which are the best conductors and radiators 
grass, leaves, metals whilst on badly conduct- 
ing substances, such as sand, gravel, glass, etc., 
little or no dew will be found. In windy 
weather, owing to the motion of the air, any 
given portion does not long remain in contact 
with the same ground, so that it is not so soon 
cooled sufficiently and less dew is deposited. 

If the temperature of the dew point is below 
the freezing-point of water, instead of dew we 
have hoar-frost, which is not frozen dew, but 
water deposited in the solid form (Scott). At 
times the frost figures seen on trees, twigs, and 
window-panes are of beautiful forms, not unlike 
the snow crystals already alluded to. 

It is stated that in some of the forests of 
tropical America the tops of the trees, by their 



Dew and Fog 169 



j cooling 



action, condense the moisture so much 
that a traveller entering finds apparently a heavy 
shower of rain falling whilst the sky overhead 
is perfectly clear (Humboldt). It has been 
estimated by Mr Dines that the average annual 
deposit of dew on the surface of the ground 
is about equal to 1*5 inches of rain. 

Owing to the fact of radiation being necessary 
for the formation of dew, its presence is an 
incidental sign of fine settled weather, though 
hoar-frosts are said to indicate rain. Thus : 

" When the frost gets into the air, it will rain " ; 
also 

" If hoar-frost come on mornings twain, 
The third day surely will have rain." 

But too much reliance need not be placed on 
such sayings. 

Fog is a phenomenon with which we in this 
country are only too familiar. Though to be 
met with at times almost anywhere, fogs are 
especially prevalent in river valleys and low- 
lying situations. Essentially the only difference 
between a cloud and a fog is that the former 
is formed in the atmosphere at some distance 
above the ground, whilst the latter is usually 
close to the earth's surface. The visible "steam" 
from a kettle is of exactly similar constitution. 
Water vapour or true steam is completely trans- 
parent and perfectly invisible, but in condensing 
it at first usually assumes the form of excessively 



170 Weather Science 

minute drops or particles, a number of which 
collect together into a cloud. Mr Aitken has 
shown that no fog or cloud can be produced 
unless there exist some solid nucleus, dust or 
other substance, round which the water particles 
may collect, and that condensation of water 
vapour in perfectly dust free air produces no 
cloud. After this condensation has taken place 
it will depend upon other circumstances as 
to what will next happen. Sometimes these 
droplets coalesce into raindrops, at other times 
hailstones are formed, whilst again the cloud or 
fog often retains its first form for a long time 
without much change. 

Fogs may arise in various ways. A mixture 
of two masses of air at different temperatures, 
giving a resultant temperature lower than is 
necessary to retain the whole of its moisture 
in the gaseous condition, part of this passes into 
the " cloud " form. A warm, damp current 
of air flowing over a chilled surface, over the 
tops of mountains, or over a cold ocean current 
also gives rise to a fog. Over rivers the amount 
of evaporation is considerable, and consequently 
the air is usually saturated. This air, on the 
smallest cooling, loses some of its moisture, and 
thus arise the mists and fogs often found near 
rivers and marshes; especially in winter time, 
when the water is usually warmer than the 
surrounding air. Of a somewhat different 
character are the sea-fogs of summer, common 



Kinds of Fogs 171 

on coast regions. The air over the sea is 
saturated with moisture from evaporation of 
the water of the latter, and when it is chilled 
by contact with colder air a fog is the conse- 
quence. " Radiation fogs " found in valleys 
and damp meadows in the evenings are thus 
explained. The air near the ground is saturated 
with moisture, and colder than that immediately 
above it. Over level surfaces there will be no 
mixture of air, the colder stratum underlying the 
warmer, but in valleys and depressions some of 
the colder air of the upper ground will flow 
down and mix with the air below, whence 
results a condensation of moisture and forma- 
tion of a fog, often very sharply defined, which 
disappears in the morning so soon as the air 
is warmed sufficiently by the sun's rays to again 
take up this moisture. 

There is *but little difference between mist and 
fog, except that in the former the particles are 
somewhat larger, and that the whole appearance 
is much damper, dry fogs being known, but 
dry mists never. Mists frequently appear over 
forests, marshy places, and on hillsides, especially 
where these are covered by trees and lofty 
herbage. 

The rainbow is an optical phenomenon familiar 
to all, whose general explanation was first given 
by Sir Isaac Newton, though a partial account, 
satisfactory so far as it went, was offered by the 
famous De Dominis, Archbishop of Spalatro, 



172 Weather Science 

nearly a century before Newton's time. Some- 
times one bow is seen, sometimes two, each of 
these presenting the well-known colours "of 
the rainbow," but in the reverse order. When 
only one bow is seen, the red arch is above 
and the violet below. The second and fainter 
bow is seen outside the primary, and in it the 
order of the colours is reversed, the violet being 
outermost and the red innermost. Between the 
two bows the sky is usually dark. These bows 
are produced by the reflection and refraction of 
the sun's rays by the drops of water forming 
the cloud against which it is seen. Sunlight 
(white) falling on the drops of water passes 
partly inside, and is partly reflected. The 
portion passing inwards is refracted (i.e., its direc- 
tion changed), but this refraction differs for the 
different kinds of radiation of which white light 
is composed. The latter contains rays of many 
kinds, those affecting the eye, when separately 
seen, causing the appearances known as red, 
yellow, green, blue, etc., their combined effect 
producing the sensation of white light. Each 
of these colours is differently refracted (or bent) 
in passing from one medium to another (e.g., 
from air to water), and in consequence the 
separate rays of white light are dispersed or 
spread out into a coloured band. The red is 
least deviated from its original cause, or is least 
refrangible, whilst violet is most deviated or 
most refrangible. 



The Rainbow 

The primary rainbow is formed of rays which 
have undergone refraction and one internal 
reflection. The angular radius of the red bow 
is 42 2', that of the violet 40 17', the inter- 
mediate colours being formed between these 
limits; the whole bow being seen nearly opposite 
to the sun's position in the sky. The secondary 
bow is formed by rays which have undergone 
two internal reflections. 1 In this the angular 
radius of the red bow is 50 58', that of the 
violet 54 9', the latter thus being the outermost. 
Owing to loss of light in reflection, etc., this bow 
is considerably fainter than the other. Rays 
which emerge parallel after three, four, and five 
internal reflections form the third, fourth, and 
fifth rainbows, but of these the third and fourth 
are formed in the part of the sky near the sun 
and cannot be seen, whilst the fifth, though it 
is formed in the opposite part of the sky, is 
so faint, after the number of reflections it has 
undergone, that it is scarcely, if ever, visible. 
When a rainbow is seen near sunrise or sunset 
it forms a complete semicircle, but usually only 
a smaller part of a circle is seen, no rainbow 
being produced when the sun's altitude is 
greater than about 42. 

Lunar rainbows are also sometimes seen, but 

1 In each case the rays suffer refraction, both in passing into and 
in coming out of the raindrops ; but the primary rainbow is formed 
of rays which have undergone one internal reflection, the secondary 
of rays which have been twice internally reflected, and which emerge 
in nearly parallel directions. 

L 



174 Weather Science 

owing to their faintness the colours cannot 
usually be distinguished. When the sun is 
shining upon the spray from a waterfall or 
fountain, an observer, with his back to the 
former, may sometimes see a complete circle 
of colour. Owing to the fact that the sun is 
not a mere luminous point, but has an apparent 
magnitude to the eye, the colours of the rain- 
bow are not pure, but somewhat mixed. When 
the sun shines through a thin cloud the rainbow 
is almost white, owing to this mixing of colours 
going on to a still greater degree. 

The large circular rings, called " halos," which 
are often seen round the moon, but less often 
round the sun, are due to refraction through, 
and reflection by the minute snow and ice 
crystals in the upper air, which compose a 
cirrus cloud. 

They are usually colourless, faint coloration 
being only occasionally observed, and should 
be distinguished from the smaller rings more 
properly known as " coronas." Most commonly 
two circles are seen, one having an angular 
radius of about 22, the other a radius of 46, 
the sun (or moon) being at the centre of these 
circles. In addition, other circles are some- 
times seen, intersecting with the primary circles, 
and spots of more brilliant light, usually reddish 
or yellowish, seen to right or the left of the 
sun's place at about the same angular distance 
of 22, which are known as parhelia, "mock 



Halos and Coronae 175 

suns," or " mock moons," paraselenes. All these 
phenomena are more common in high latitudes 
than in our own, and during winter rather than 
summer, as was to be expected from their pro- 
duction by the refraction of light through ice 
crystals. The appearance of a halo is a very 
certain sign of unsettled, stormy weather, but 
the phenomenon is of much less frequent occur- 
rence than that of a corona. 

The latter is a coloured ring, or series of rings, 
often seen round the moon when the sky is 
covered with much cloud, but its visible appear- 
ance round the sun is much more rare, being 
usually too faint to be detected. 1 The colours 
of this corona are arranged in the same order as 
those of the rainbow, violet inside and red out- 
side, but they are usually faint and much less 
pure. When several rings are seen the inner- 
most one will have a diameter of about 3 or so, 
the next will be about double this, and the third 
three times that of the first. 

These phenomena are due to "diffraction," 
or bending of the rays of light by a number 
of minute particles (the water-drops forming 
the cloud) of more or less nearly equal size. 
The diameter of the corona depends upon the 
size of these droplets and varies inversely there- 
with, growing smaller when these are increasing 

1 This optical effect, or corona, of course, is in no way connected 
with the true solar Corona only seen at the times of a total eclipse. 
It is somewhat unfortunate that the same name should be used for 
two so distinct phenomena. 



176 Weather Science 

in size and larger when they are decreasing. 
Thus it is sometimes possible, by watching the 
change in size of a corona, to infer whether 
the coming weather is likely to be rainy, or the 
reverse. When the corona is getting smaller 
the water particles are uniting and will sooner 
or later be condensed and precipitated in the form 
of rain ; when the diameter of the corona is 
increasing, the particles are getting smaller and 
evaporating. This appearance has been artifici- 
ally reproduced by Frauenhofer and others. By 
looking at a luminous object through a glass 
covered with fine grains, such as those of 
lycopodium powder, a number of luminous 
rings will be seen surrounding the object ; and 
an instrument called the eriometer was invented 
by Dr Thomas Young to measure the diameter 
of Very small objects by means of the size of the 
rings produced by them round luminous objects, 
the radii of these rings being inversely pro- 
portional to the diameter of these small 
particles. 

Similar appearances may be seen by looking 
through the meshes of a fine handkerchief at a 
gas lamp, or through a dimmed spectacle glass 
on a wet evening! 

In Arctic Regions coloured circles of light 
are sometimes seen round the shadows of the 
observer's head, or other objects. 

The mirage is an optical phenomenon, more 
common in hot countries than in temperate 



The Mirage 177 

regions, though it has been often observed by 
Scoresby and others in high latitudes also. 

The observer sometimes sees the inverted image 
of distant objects and the sky, reflected from the 
ground as from a lake. In hot, sandy districts 
the lower strata of air are sometimes so heated 
that up to the height of a few feet the density 
is less below than above. Rays of light from 
distant objects entering these layers of rarefied 
air obliquely downwards become bent upwards 
more and more till they fall on a stratum where, 
instead of being refracted, they suffer total 
internal reflection. Thus they will be bent 
upwards more and more after passing through 
the denser layers above, and will reach the eye 
as if they came from a point as far below the 
reflecting layer as the objects of which they are 
images are above it. Sometimes an object is thus 
seen inverted, whilst other rays which do not 
pass down into the reflecting layer enable the 
observer to see it in the upright position, the 
combination thus seeming like object and image 
reflected in a lake of calm water. 

A similar mirage may sometimes be seen 
across lakes on tranquil autumn evenings. 
Here it is the water that heats the lower 
layers of air, and thus renders them "totally 
reflecting " (Stewart). Sometimes in the Arctic 
Regions inverted images of ships are seen in 
the air, at a time when they are too far away 
to be directly seen. 



178 Weather Science 

It is recorded that Scoresby once observed 
the image of his father's ship in the air at a 
time when she was more than 30 miles off. 
The effect in this case is due to the cooling of 
the lower air. The rays passing obliquely 
upwards from the objects into the rarer layers 
above are more and more bent downwards until 
they suffer total reflection, and the object is seen 
apparently up in the air at a great distance. 
Some years ago, at Dover, ships close in to the 
French coast were thus distinctly seen from the 
English side of the Straits (Stewart). 

The phenomena of looming, the abnormal 
elevation of distant objects, inverted images 
seen above the objects, etc., are all due to 
optical effects of a similar kind. It has been 
elsewhere mentioned that the blue colour of the 
sky is probably due to the scattering of light 
from small suspended particles in the air. 

Tyndall supposed that these might be minute 
water particles, whilst Lord Rayleigh has sug- 
gested that fine salt particles, meteoric dust, 
etc., and even the oxygen of the atmosphere, 
may be effective in producing this coloration. 

The red colour of the sun and moon near 
rising and setting (and also during foggy 
weather), the wondrous tints of the clouds at 
dawn and sunset, are also due to selective 
absorption and diffraction. Sunlight, composed 
of all the " colours of the rainbow," is more or 
less absorbed in passing through the atmosphere, 



Colours of Clouds, etc. 179 

the vapours and dust particles of which exert a 
much greater action than the pure gases. This 
action is greater upon the shorter blue and 
violet rays than on the longer (red and yellow) 
ones, and most of the absorbent particles are 
found in the lower layers of the air. Thus 
sunlight which has travelled through a consider- 
able thickness of the atmosphere will lose a 
greater proportion of its blue and violet rays than 
of the longer ones, and consequently these 
latter predominating, the remaining light will 
be yellowish or reddish. The late Professor 
Langley was of opinion that if we could ascend 
above the atmosphere, the sun would be seen 
as " blue " or bluish (more nearly like the colour 
of the electric light) rather than white, as we 
ordinarily see him. 

An extra thickness, as at sunrise and sunset, 
and an unusual amount of dust and other 
impurities in the air (as in a November or 
January fog in London), removes almost all the 
other rays and leaves the sun only visible as 
a fiery red ball. 

The cloud colours are similarly due to 
selective "sifting," as it were; the shorter 
rays being absorbed, the longer more or less 
reflected. When sunlight falls upon the surfaces 
of the upper clouds, the violet and blue rays 
are absorbed, but much of the yellow and red is 
reflected, and reaches our eyes. The eruption 
of Krakatoa in 1883 throwing an enormous 



180 Weather Science 

mass of matter into the air, dust particles 
probably travelling for thousands of miles, and 
remaining many months in the atmosphere 
before subsiding, it was to the presence of these 
particles that the brilliant sunsets experienced 
at various places during 1883 and 1884 was 
attributed, though as brilliant cloud effects have 
been observed at other times also. Mr John 
Aitken, to whom is due the recognition of the 
importance of dust in Nature, and who, more 
fitly perhaps than Ruskin, may be considered 
the founder of the " Ethics of Dust," showed 
that minute motes or particles are everywhere 
present, even in the purest and clearest air 
remote from towns. These minute particles 
catching and scattering the sunlight, are the 
agents by which the atmosphere is illuminated, 
and if the air were free from them, the sky 
would be seen perfectly black except where the 
sun or a star was to be found. He considers 
the blue of the sky to be due to the scattering 
action of the dust motes in the higher layers, 
as also the red tints of sunrise and sunset, and 
the twilight generally. 



Heat Transference 181 



CHAPTER X 

METHODS OP HEAT TRANSFERENCE OCEAN CURRENTS THE GULF 

STREAM INFLUENCE ON TEMPERATURE AND OTHER WEATHER 

CONDITIONS THEORIES AS TO ITS ORIGIN, ETC. THE HUMBOLDT 

OR PERUVIAN CURRENT COOLING EFFECT CHINA CURRENT 

tf KURO SI WO " CURRENTS OF THE INDIAN OCEAN AGULHAS 

CURRENT POLAR CURRENTS IDEAS AS TO THEIR CAUSES. 

THERE are three great methods whereby heat 
is conveyed from a body radiating it, to others 
radiation, conduction, and convection. Radiant 
heat is a form of energy, differing only from 
light in the greater length of its waves, and 
travels, apparently without the help of a 
material medium (unless the ether be such), 
from the sun and stars throughout space to 
our earth. Its course undergoes a slight re- 
fraction or change of direction by passing 
through the atmosphere, and it is to a small 
extent absorbed or converted into sensible heat 
by this medium, but a large part of it reaches 
the lower regions and warms the ground and 
the waters of the seas. By a much slower 
process some of this heat received by the 
ground is conducted downwards, and in other 
directions, in the same way that a poker with 
one end plunged in a flame gets gradually hot 



1 82 Weather Science 

all through. This process is called conduction, 
and is almost confined to solids. The effect of 
heat when absorbed being in general to cause 
an expansion of the substances receiving it, 
they become specifically lighter. The particles 
of a solid are held together by cohesion, but 
those of a fluid, liquid or gas, are not thus 
restrained. Consequently, heated fluid particles, 
if the heat be first imparted from below, 
become lighter and rise, their place being taken 
by the heavier particles above, which sink, and 
thus ascending currents of hot particles and 
descending ones of colder are set up. This 
process is known as convection, being a bodily 
transference of particles carrying heat with 
them, part of which is imparted by radiation 
and sometimes by conduction to other sub- 
stances with which they come in contact. 

Thus arise convection currents, of which the 
ocean currents, and the great air currents, equa- 
torial and polar, etc., are well-known examples 
on the largest scale. 

The motions of water through the ocean in 
definite directions being an actual transference 
of the particles of the fluid must be distinguished 
from tidal and other waves, which, though 
setting up motions which travel sometimes all 
round the globe, do not usually cause any 
bodily transference of matter, but merely a 
slow up and down motion of individual particles. 
The particles of water rise and fall to a greater 



Atmospheric Tides 183 

or less degree, but do not change their mean 
position; it is merely the transference of the 
wave form that takes place, so that we may 
regard a wave as being a means for the 
motion of energy from point to point, whilst 
a current transfers matter also. It sometimes 
happens, as at the mouth of large rivers, that 
the small oscillatory tidal motion of the ocean 
is transferred into a bodily motion of large 
masses of liquid, which travel upwards towards 
the source, and then down again periodically 
(usually twice in every day), thus the tidal 
wave in this case becomes a current. 

The tides of the ocean are of extreme im- 
portance in navigation and many other ways, 
but their detailed study scarcely belongs to our 
subject. On the other hand, the tides of the 
atmosphere, or at least that small portion of it 
which has alone hitherto been carefully studied 
or accessible to our study, are, so far as known, 
small and insignificant, in most cases masked 
by far larger irregular forms of motion, though 
in tropical regions the daily "barometric tide" 
due to the sun, and no doubt to some extent 
the moon also, is a sufficiently marked phe- 
nomenon, at least, during settled weather, any 
interruption to which is a sure sign of a change. 
Indeed, so regular is the daily rise and fall of the 
barometer (not amounting anywhere to more than 
0-12 inch), that it is said that in India the time of 
day can be told by the reading of the barometer. 



184 Weather Science 

But the importance of ocean currents is such 
that some account of the principal of these must 
now be given, as well as their effect upon 
climatic conditions. 

Some currents, such as the Gulf Stream, the 
best known of all, convey heat from the equa- 
torial regions towards the extra-tropical latitudes ; 
others, such as the Humboldt current of the 
Southern Pacific, help to render the climate of 
the regions towards which they blow more 
foggy and colder than would otherwise be the 
case. 

The words of Maury ("Physical Geography 
of the Sea") may fittingly serve as an intro- 
duction : 

" There is a river in the ocean. In the severest 
droughts it never fails, and in the mightiest floods 
it never overflows. Its banks and its bottoms 
are of cold water, whilst its current is of warm. 
The Gulf of Mexico is its fountain, and its 
mouth is in the Arctic Seas. It is the Gulf 
Stream. There is in the world no other such 
majestic flow of waters. Its current is more 
rapid than the Mississippi or the Amazon, and 
its volume more than a thousand times greater." 

Circulating thus from the Gulf of Mexico, 
it passes out into the Atlantic Ocean ; it pours 
out through the Straits of Florida, as a stream 
of hot water, whose temperature is at least 10 
above that of the surrounding ocean through 
which it flows, at a rate of over 6 feet per second, 



The Gulf Stream 185 

having a width of about 30 miles when passing 
the Straits of Bernini. It gets gradually wider 
as it flows northwards (its depth off Cape 
Hatteras being about 700 feet), till off the coast 
of Newfoundland it has a width of 320 miles, 
after travelling nearly 2,000 miles from the 
Straits of Florida, its speed meanwhile having 
fallen off to one-third of its original value. It 
flows thence north-eastwards towards the Azores, 
where it divides, one portion going past these 
islands, and to the north of Norway ; the other 
part bending more to the right, passing the 
coast of Portugal, and going southwards towards 
the Cape Verde Islands, whence it runs back 
almost due westward to the West Indies. 

From the Straits of Bernini the course of 
the Gulf Stream is as nearly as possible the 
arc of a great circle, very nearly the course that 
would be taken by a cannon-ball, could it be 
shot from these Straits to the British Islands 
(Maury). The waters are of an intensely blue 
colour, whose temperature on a winter's day, off 
Cape Hatteras, and even as high as the Grand 
Banks of Newfoundland, in mid-ocean, is some- 
times 20 or even 30 above that of the ocean 
near by. It flows through the Straits of Bernini 
with a temperature exceeding 80 F. ; but this 
falls off as it proceeds northwards, yet it 
everywhere exerts a beneficial influence on the 
neighbouring lands. 

As an instance of the Gulf Stream influence 



1 86 Weather Science 

upon British climate, and the contrast between 
our temperatures and those of regions where 
its action is not felt, may be mentioned the 
opposite cases of the harbours of Liverpool and 
St. John's, Newfoundland, instanced by the late 
Mr Redfield. 

"The latitude of St. John's (48 N.), is 5 
less than that of Liverpool (53 N.), but whilst 
in 1831 the harbour of the former was closed 
with ice as late as the month of June, who 
ever heard of the port of Liverpool being 
closed with ice even in the dead of winter?" 

It is even stated that the ponds in the 
Orkneys (latitude 60) are not frozen in winter 
(Scott) ; another instance of the " grand heating 
apparatus " of this " stream." 

Not only does the Gulf Stream convey heat 
to regions where the sun shines less powerfully 
than it does in the Gulf of Mexico, but by its 
carrying off the heated waters northwards, it gives 
place to cooler currents through the Caribbean 
Sea, these helping to moderate the excessive heat 
of the countries around. Different estimates 
of the effect of the Gulf Stream have been 
made by the late Dr Croll, the late Rev. Professor 
Haughton, and others. Dr Croll finds that the 
Gulf Stream conveys into the Atlantic about 
one-fifth of the "total heat" found in it, but 
this seems too extravagant an estimate. Taking 
the temperature of the surface at 56 F., and 



Heating Effects 187 

the "absolute zero" as -460 F., we have 56 + 
460 = 516, one-fifth of which is 103, and if 
we subtract this from 56, we have 56-103 = 
-47 F., or nearly 80 below the freezing-point 
of water, as the temperature of the Atlantic, 
apart from the influence of the Gulf Stream. 
Mr Scott's calculation ("Meteorology," p. 301), 
giving -3 as the resulting temperature, though 
too high, unless we assume with him the tem- 
perature of " space " to be more than 200 F. 
above the absolute zero (-239), shows this suffi- 
ciently well, but the result is yet more striking 
with the figures I have taken. On the other 
hand, Dr Haughton found a much smaller effect, 
a scarcely perceptible action during the summer 
months, perhaps a slight lowering, but a very 
sensible increase of temperature for the winter, 
amounting for latitude 60, to about 37*0 F., 
and over 40 F. for latitude 70, its value for 
our latitudes (50) being 21-7. 

The motion of the current off our own islands 
is very slow, estimated at not more than 1 inch or 
so per second, but evidence of this motion is said 
to be afforded by the occasional discovery of 
West Indian fruits, beans, etc., on the western 
coasts, thrown up by the waters of the ocean. 
The time required by the waters of the Gulf 
Stream to flow from Florida to Western Europe 
has been estimated at between five and six 
months, and the whole time required for a 
particle to make a complete circulation back 



1 88 Weather Science 

again into the Gulf of Mexico after coming 
to Europe would be about two years and ten 
months (Scott). 

With regard to the influence of the Gulf 
Stream upon climate, we have already indicated 
some of the more prominent thermal effects, 
about whose magnitude, however, some difference 
prevails, some recent authorities perhaps being 
as much inclined to underrate these as others 
formerly overrated them. The dampness of 
Ireland in particular, and the British Islands in 
general, as compared with the climate of other 
parts of Western Europe, may be partly set 
down to its credit, but only so far as the rain 
of the south-westerly winds is produced through 
its agency. With regard to its influence upon 
the meteorology of the ocean through which it 
flows, a few remarks must here suffice. The Gulf 
Stream is the great " weather -breeder " of the 
North Atlantic (Maury). Various gales of wind 
sweep along it from time to time, and " sailors 
dread storms in the Gulf Stream more than they 
do in any other part of the ocean." The stream 
current flowing in one direction whilst the wind 
is blowing in another, creates a very disturbed 
state of the water. Many of the most destruc- 
tive storms appear to owe their origin to the 
irregularity between the temperatures of the 
Gulf Stream and those of the neighbouring air 
and water. Another remarkable directive action 
has been attributed to this current, whose general 



Theories as to its Cause 189 

effect is so beneficial. Storms of greater or less 
magnitude that take their rise on the African 
coast have been shown to make straight for 
the Gulf Stream ; on joining it they have then 
changed their course, travelled with the current, 
and recrossed the Atlantic, carrying disaster in 
their train. 

Many theories have been propounded from 
time to time as to the cause of this great 
ocean current. Early writers, regardless of the 
fact that the volume of the Gulf Stream many 
times exceeds that of any river, supposed that 
the Mississippi River, flowing into the Gulf of 
Mexico, became the "father" of the Gulf Stream; 
but this was conclusively disproved by the enor- 
mous volume of the latter, and the fact that 
its water is salt, whilst that of the river, its 
supposed origin, is fresh. Franklin's view was 
that the Gulf Stream is the escaping of the 
waters that have been forced into the Caribbean 
Sea by the trade winds, and that it is the 
pressure of those winds upon the water which 
forces up into that sea a " head " for the stream. 
This view seems the most probable one as to 
the origin of this current, but objections have 
been raised by Maury and others as to the 
adequacy of the wind pressure to alone give 
the necessary initial velocity, though they do not 
deny that this must be an important factor, 
combined with other agencies; amongst which 
the late Commander Maury was inclined to 

M 



i go Weather Science 

consider the difference of saltness prevailing 
between the equatorial and polar seas, on the 
one hand, the Caribbean Sea and the Gulf of 
Mexico, "with their waters of brine," on the 
the other, the great polar basin, the Baltic, and 
North Sea, with their little more than brackish 
waters. Another cause of circulation is the 
difference of temperature between the inter- 
tropical and polar seas. 

" In one set of these sea-basins the water is 
heavy, in the other it is light. Between them 
the ocean intervenes, but water is bound to seek 
and to maintain its level, and here, therefore, we 
unmask one of the agents concerned in causing 
the Gulf Stream." MAURY. 

Amongst other currents whose influence on 
the climate of the regions towards which they 
flow, though less marked than the Gulf Stream, 
is considerable, may be mentioned the well-known 
Humboldt current of the South Pacific, so 
named from the "great and good man" who 
was the first to make it known, the polar 
current of the North Atlantic, the warm current 
of the Indian Ocean, flowing south midway 
between Africa and Australia, and the China 
Gulf Stream " of the Northern Pacific, whose 
course is not quite so well known, the Agulhas 
current of the Indian Ocean, etc. 

The Humboldt current flows northwards from 
the South Pacific or the Southern Ocean, past 



Humboldt Current 191 

the western coasts of Chili and Peru, bringing 
up colder water than that of the surrounding 
sea, almost as far as the Equator. Its effect is 
most preceptible, however, between 20 and 30 S. 
latitude. To a certain extent it mitigates the 
otherwise rainless climate of Peru and renders 
the latter much more salubrious than would 
else be the case. 

" The climate of this inter-tropical republic is 
thus made one of the most remarkable in the 
world; the Andes with their snowcaps on the 
one side of the narrow slopes, and the current 
from the Antarctic regions on the other, cause 
the temperature to be such that cloth clothes are 
seldom felt as oppressive during any time of the 
year, especially after nightfall." MAURY. 

Between the region of this current and the 
Equator is an area known as the "desolate 
region." Few birds are ever found here ; the sea- 
bird that joins the ship as she clears Australia 
will, it is said, follow her to this region and then 
disappear ; " even the chirp of the stormy petrel 
ceases to be heard here," the whale is seldom 
seen, and the albatross and Cape pigeon are no 
longer found. There appears to be a warm 
current flowing from the inter-tropical regions 
of the Pacific, midway between the American 
coast and the shore lines of Australia, whilst 
further to the northward is to be found the 
China current, or " Gulf Stream of the Pacific," 



192 Weather Science 

as it is sometimes called, or the Kuro-siwo of 
the Japanese. This current of warm water is 
of considerably greater volume than that of its 
namesake in the Atlantic, and serves to mitigate 
the severity of the winter on the shores of 
Alacka and British Columbia. As with the Gulf 
Stream, there is a counter-current of cold water 
between it and the shore (Maury). 

Thus a parallel may be drawn between the 
opposite hemispheres, the climates of the Asiatic 
coast of the Pacific corresponding with those 
of North America along the Atlantic, China 
answering to the United States, the Philippines 
to Bermuda, Japan to Newfoundland, whilst 
Columbia, Washington, and Vancouver corre- 
spond to Western Europe and the British 
Islands. 

"The climate of California (State)," says 
Maury, " resembles that of Spain ; the sandy 
plains and rainless regions of Lower California 
remind one of Africa, with its deserts between 
the same parallels, etc. The North Pacific, like 
the North Atlantic, is enveloped, where these 
warm waters go, with mists and fogs. . . . 
The Aleutian Isles are almost as renowned for 
fogs and mists as are the Grand Banks of 
Newfoundland." 

The South Pacific current, flowing from the 
eastward of New Guinea south towards Australia 
and thence again eastward, appears to have been 
first detected by the late Commander Maury, 



Currents of the Indian Ocean 193 

who also suspected the existence of smaller 
equatorial currents in this ocean. Both in the 
North and South Atlantic there are to be found 
regions where little or no current is perceptible, 
and where, as a consequence, drift-wood, sea- 
weed, and other debris collect. This region in 
the Atlantic is known as the "Sargasso Sea," 
from the quantity of gulf -weed, sargassum 
bacciferum, a vegetable growth found covering 
many leagues of the surface of the ocean. The 
similar region in the South Atlantic contains 
less weed, but is still known by analogy as 
the Southern Sargasso Sea, whilst Maury and 
others have asserted the existence of a Sargasso 
Sea in the Pacific also. 

The currents of the Indian Ocean are governed 
mainly by the prevailing winds north of 
the Equator; during the "winter" half-year 
the currents run generally westward under the 
influence of the north-east monsoon. For the 
rest of the year (April to October), "summer," 
under the influence of the south-western 
monsoon, the waters flow in the opposite 
direction. 

South of the Equator the great Mozambique 
or Agulhas current, an immense volume of warm 
water, flows at first in a south-easterly direction 
till it rounds the southern coast of Africa, and 
then passes into the Atlantic, after clearing 
Cape Agulhas. This current, where it meets 
the south-easterly current, produces a consider- 



194 Weather Science 

able elevation of the surface temperature of the 
sea, and, like the Gulf Stream, in its neighbour- 
hood violent storms and high seas are often 
experienced. (The early name of the Cape of 
Good Hope, given it by Diaz, was the "Cape 
of Storms," but this ill-omened appellation was 
changed by King Emanuel of Portugal to the 
happier name it bears at present.) Another 
cold current flowing eastwards passes south of 
Australia and thence into the South Pacific ; 
a small part dividing from the main stream 
turns northwards towards the Equator, lowering 
the temperature of the west Australian coast 
regions. 

Of polar currents the best known is the 
American Arctic current, whose course south- 
wards from Baffin's Bay, following closely the 
outline of the American coast, influences the 
temperature of those regions as far as latitude 
40, and serves to accentuate the difference of 
climate between corresponding latitudes in 
Europe and North America, the former being 
warmed by the influence of the Gulf Stream, 
the latter cooled by this Arctic current. 
Behring's Strait "being too shallow to admit 
of mighty undercurrents or to permit the intro- 
duction from the polar basin " of much water, 
the Northern Pacific has no corresponding polar 
current. Similarly, there being no escape for 
the warm Pacific waters into the polar basin, 
they are turned " down through a sort of North 



Polar Currents 195 

Sea along the western coast of the continent 
(of America) towards Mexico " (Maury). They 
appear as a cool current, and give freshness and 
strength to the "cooling sea-breeze of the 
Californian coast in summer time." 

The polar currents of the Southern Hemi- 
sphere are of extensive influence, and form 
important features of the physiography of that 
part of the world. The Peruvian or Hum- 
boldt current has already been mentioned, but 
there is also a well-marked stream in the South 
Atlantic impinging on the south-western coast 
of Africa, and indeed it is sometimes stated 
that almost all the surface water between 
the Antarctic Circle and latitude 45 S. is drift- 
ing northwards and eastwards. 

Thus by these currents, flowing in various 
directions, some carrying water hotter than the 
surrounding sea, others carrying cold water and 
sometimes icebergs, there is a constant circula- 
tion of the waters of the ocean. " Westerly 
currents generally flow round the earth on low 
latitudes, and counter - currents flow eastwards 
close to the Equator" (Scott). 

Currents of hot water flow Polewards from 
the Equator, cold currents flow from the Poles 
towards low latitudes, and thus produce the 
compensation necessarily wanted to maintain the 
general level of the ocean surface, to prevent a 
permanent heaping up of the waters in one 
place, or a defect in another. 



196 Weather Science 

With regard to the cause of these currents, it 
is evident that differences of specific gravity are 
all-sufficient, but much speculation may be in- 
dulged in as to whether these differences arise 
more largely from differences of temperature or 
of saltness. The river waters running into the 
sea tend to diminish its saltness, whilst evapora- 
tion produces the reverse effect, the former 
lowering and the latter raising the specific 
gravity : an increase of temperature acts similarly 
to the first ; a fall of temperature, by radiation 
or otherwise, increases the specific gravity. On 
the whole, notwithstanding these various 
actions, the mean specific gravity of the water 
of the sea undergoes no change, though in 
certain land-locked basins, such as the Baltic, 
there is to be found brackish rather than salt 
water. 

Since the water in polar seas contains a 
slightly smaller proportion of saline matter, etc., 
than the average, owing to the melting of ice- 
bergs, whilst the greater evaporation at the 
Equator causes the proportion of salt to be 
larger, the influence of greater or less saltness 
tends in the contrary direction to that of 
temperature. The lower temperature of the 
polar waters increases their specific gravity, 
which the greater freshness would diminish, 
whilst the waters of the equatorial seas, rendered 
lighter by the influence of the sun's heat, 
are made denser by concentration of their 



Moderating Effect of Current 197 

salts owing to the rapid evaporation at the 
surface. 

Thus by means of these warm and cold 
currents the ocean acts as a most important 
regulator and moderator of terrestrial climates. 
It carries a very large part (perhaps half) of the 
sun heat falling on the tropical zones to higher 
latitudes, there warming them, and conversely 
by means of the cold polar currents it serves to 
modify the otherwise excessive heat of many 
parts of the tropics. On account of the great 
specific heat of water, the latter is but slowly 
raised in temperature by a considerable accession 
of heat, and cools slowly on its withdrawal. 
"All the rivers run into the sea, yet the sea 
is not full," said the old Hebrew writer, and 
supplied the answer : " Unto the place whence 
the waters comes, thither do they return," 
evaporation making up for the addition. An 
enormous amount of heat, rendered latent in 
the process of evaporation and given out again 
in condensation, is conveyed from place to 
place, having been thus indirectly derived from 
the sea. The great evaporation of water in 
the Red Sea raises its density, and would 
lower the level of its surface by from 10 to 
20 feet in a year were it not for a current of 
fresher water from the Indian Ocean pouring in 
through the Straits of Bab-el- Mandeb. Since 
there is no perceptible change in the density 
of the water, notwithstanding this excessive 



198 Weather Science 

evaporation, it would seem that there must be a 
deep undercurrent of salter water back to the 
Indian Ocean, and an underflowing outward 
current in the Mediterranean, below an in- 
flowing upper current through the Straits of 
Gibraltar (Mill). 



Atmospheric Electricity 199 



CHAPTER XI 

ATMOSPHERIC ELECTRICITY INSTRUMENTS QUADRANT ELECTROMETER 
PORTABLE ELECTROMETER THUNDERSTORMS LIGHTNING CON- 
DUCTORS ST ELMO'S FIRE ELECTRIC "HUM" THE AURORA 

MAGNETIC STORMS MAGNETIC AND TRUE BEARING. 

THE atmosphere is ordinarily found to be more 
or less in a condition of electrical instability ; 
in fine weather usually positively charged to a 
greater or less degree, in broken weather it is 
often negatively charged. In addition to this 
we have the phenomena of thunderstorms, and 
the aurora, which, according to its appearance, 
is known as " Borealis " or " Australis " respec- 
tively, is seen in high northern and southern 
latitudes, but rarely near to the Equator. Our 
knowledge of the phenomena of atmospheric 
electricity has been much extended of late 
years by the work of Lord Kelvin, whose 
electrometer and water-dropping collector are 
most useful for their observation. Quetelet at 
Brussels and Everett in Canada have made 
observations with these instruments and obtained 
some interesting results. The former found that 
the daily indications during fine weather showed 
two maxima in summer, at 8 A.M. and 9 P.M. 



200 Weather Science 

respectively, and in winter at 10 A.M. and 6 P.M., 
the two minima being about 3 P.M. and mid- 
night, the intensity of electrification being 
much greater in winter than in summer. At 
Kew the maxima and minima occur at somewhat 
different hours, whilst in Paris it is stated that 
M. Mascart finds only one maximum just before 
midnight. The electrification diminishes from 
sunrise till about 3 P.M., when it reaches a 
minimum, after which it rises to nightfall 
(Thompson). (Messrs Ebell and Kurz at 
Munich find two daily maxima and two minima 
for the summer, but only one for winter, for 
both positive and negative ionisation). 

As a general rule, the air over the earth's surface 
is positively electrified with reference to the sur- 
face of the ground, the lower air is almost a non- 
conductor, whilst the upper regions, where the air 
is rarefied, conduct an electric charge somewhat 
in the manner of the rarefied gases within a 
Geissler tube, and are usually positively charged. 
Thus the lower air acts as the dielectric between 
the upper charge of positive electrification and 
the negative charge on the earth's surface. By 
measurement at different heights it has been 
found that the potential of the air gets higher 
as we ascend. Lord Kelvin found that the rise 
of potential was about 40 volts per foot (or 1*3 
volts per centimetre) for Aberdeen (Thompson). 
Though, as we have said, the air is generally 
positively electrified in fine weather, during rain 



Electroscope Volta's Method 201 

and broken weather the electrification is often 
negative, and undergoes rapid changes. A 
definite change in the electrical conditions 
usually accompanies a change of weather 
(Thompson). 

The simplest instrument for detecting the 
presence of a charge, and its sign, whether 
positive or negative, is the electroscope, such 
as the Gold Leaf or Bohnenberger's, described 
in most books on electricity. As, however, 
indications are very feeble near the ground, the 
earlier observers affixed an insulated pointed rod 
to the top of the electroscope, or shot up into 
the air an arrow connected by a wire with the 
apparatus. Volta's method was either to use a 
slow - burning match attached to the top of a 
long metal rod, or else he employed a flame burn- 
ing at a height. The particles discharged (and 
heated air driven off) from the flame thus serve 
to equalise its potential with that of the sur- 
rounding air; each particle as it leaves carries 
with it a small charge, + or - as the case may be, 
till the potentials of the rod and the air around 
are the same. The same thing is also effected 
by means of Lord Kelvin's "water dropping" 
collector. This is an insulated copper cistern, 
having a nozzle protruding into the air. This 
can being filled with water and the tap turned 
slightly on, so that a small stream of water 
pours gently out in drops, in a very short time 
its potential will be found to be the same 



202 



Weather Science 



as that of the air at the point of the nozzle. 
During frosty weather pieces of blotting-paper 
steeped in a solution of nitrate of lead, dried 
and rolled into matches, which when ignited 
smoulder slowly, are used instead. For examining 
and measuring the charge either Lord Kelvin's 

quadrant electrometer 
or his portable electro- 
meter are the most con- 
venient instruments. 
M. Quetelet's observa- 
tions, already alluded 
to, were made with 
Peltier's electrometer. 
The Quadrant Electro- 
meter designed by Lord 
Kelvin for the accurate 
measurement of small 
charges consists essen- 
tially of a needle made 
of a thin flat piece of 
aluminium, hung hori- 
zontally by a thin wire thread, or by two fine 
wire or silk threads, whose distance apart can 
be varied slightly. The needle carries at its top 
a small light mirror, whereby its movements can 
be accurately observed by watching the image 
of a scale reflected in it. 

This needle swings inside a brass box, in which 
are four quadrant pieces, placed just below 
the needle without touching it or one another. 




Quadrant Electrometer. 



Quadrant Electrometer 203 

Opposite quadrants are joined by wires. Then 
if, say, quadrants 1 and 3 are slightly -has com- 
pared with 2 and 4, the needle will move 
from the former more nearly towards the latter, 
if positively charged, and the reverse way if 
negative, according to the usual law of electrifi- 
cation, positive charges being repelled by positive 
and attracted by negative "like repels like, and 
unlike attracts unlike." When the differences 
of potential are small the deflexions, also small, 
will be very nearly proportional to these differ- 
ences, and the instrument is sufficiently delicate 
to show a difference of potential between the 
quadrants of less than / n th that of the Daniell's 
;ell. A small charge must be imparted to the 
needle by means of an electrophorus or other- 
wise. One of the electrodes or wires, joining one 
pair of quadrants, is then joined or connected 
by a wire to the water-dropping collector, the 
other, joining the opposite pair, is connected 
with " earth " by means of a wire to a gas-pipe, 
or otherwise. Additional refinements, such as 
the use of two sets of quadrants instead of one, 

" replenisher " to keep up the charge, and 
pumice soaked in sulphuric acid to absorb the 
moisture and keep the interior of the instrument 
dry, are often employed, but the needle and 
quadrants are the essential features. 

The portable electrometer is a form of the 
"attracted disc" instrument, originally designed 
by the late Sir W. Snow Harris, but improved 



2O4 Weather Science 

by Lord Kelvin. This consists of a small glass 
Leyden jar, having inside it an attracted disc and 
guard plate, which are placed in communica- 
tion with a condenser to keep them at a known 
potential. The condenser is provided with a 
gauge, itself also an attracted disc, to indicate 
when it is charged to the right potential, and 
with a small influence machine to act as a 
replenisher and increase or decrease the charge if 
necessary. As with the quadrant electrometer 
pumice-stone soaked in sulphuric acid helps to 
keep the interior of the instrument dry, and 
thus prevent too rapid loss of charge. 

In addition to the slight continuous indica- 
tions of electrification already referred to, the 
phenomena of thunderstorms, and perhaps the 
more violent atmospheric disturbances accom- 
panying " water-spouts " and hailstorms, come 
under the heading of atmospheric electricity. 
Benjamin Franklin, by sending up a kite with 
an iron point during the passage of a storm, 
found the wetted string to conduct electricity 
downwards, and was enabled to draw an abund- 
ance of sparks, which could be used to charge 
Leyden jars and produce all the electrical 
effects associated with the " static " electricity 
of the frictional machine, thus demonstrating the 
identity of the electricity of a thunder cloud 
with that obtained from the latter. In France 
Dalibard, at Marly, near Paris, erected an 
insulated iron rod, whence he drew sparks from 



Theories of Thunderstorms 205 

the clouds during a storm, and Richmann, at 
I St Petersburg, experimenting with a similar 
apparatus, was struck by a spark and killed 
, on the spot. 

There is still a good deal of uncertainty as 
to the cause of the excessive accumulation of 
charge shown in these vast developments of 
atmospheric electricity, and new " theories of 
thunderstorms " continue to make their appear- 
ance from time to time in the scientific and 
popular magazines, one of the most recent being 
outlined in Nature for 17th November 1910. 
This is due to Dr Simpson, of the Indian 
Meteorological Department. He supposes that 
upward currents of air prevent rain that would 
otherwise be deposited, from falling. The rain 
drops grow " through cycles of growth," then 
break up with separation of electricity, until 
their charge is so great as to produce a gradient 
of more than 30,000 volts per centimetre. Then 
the lightning flashes, and the accumulation 
is neutralised over a limited area ; the process 
goes on again, another flash takes place, and 
so on, till equilibrium is finally attained and the 
storm ceases. 

An outline theory of thunderstorms, favoured 
by Dr S. P, Thompson, may be here briefly 
sketched. The clouds are usually more or less 
charged with electricity, mainly derived from 
evaporation going on at the earth's surface. 
The minute particles of water vapour floating 

N 



2o6 Weather Science 

in the air become more highly charged. As 
they fall and coalesce 1 the strength of their 
charges increases ; for eight small spherical drops 
equally charged will unite into another sphere 
of twice the radius (2 3 = 8), but having eight 
times the quantity of charge, and therefore the 
potential of the larger sphere will be f = 4 times 
that of each original smaller one. Thus a mass 
of cloud consisting of such drops will rise in 
potential by their coalescence, the electrifica- 
tion at the lower surface becoming greater 
and greater, whilst the earth below becomes 
oppositely charged by influence; the layer of 
air between acting as the dielectric, the whole 
arrangement will become, as it were, a kind of 
condenser. After a time the difference of 
potential will be so great that the air between 
will give way, and a disruptive charge take place 
along the path of least resistance. Other dis- 
charges will also take place, since the first will 
only discharge the electricity at the surface of 
the cloud, and the other parts of the cloud will 
next react upon the discharged portion, so that 
a series of flashes will result. The discharge, 
known as the lightning flash, may be of one 
or other of three kinds: (1) forked or zig- 
zag lightning; (2) sheet lightning; (3) ball or 
globular lightning. Of these forms the most 
common is the first, the zigzag form, less evident 

ivA^l 1 ^ 16 '^ b y experiments with electrified water jets, found 
that slightly charged drops have a tendency to coalesce (1879). 



Forms of Lightning 207 

from photographs than it appears to the un- 
aided eye, and is probably due to the presence of 
obstacles, solid particles or local electrification at 
various points, so that a crooked path, though 
longer than the direct one, is a line of least 
resistance. Sheet lightning, usually seen on 
the horizon at night time without thunder, is 
the reflection on the clouds of flashes from a 
distant storm. The third form, globular or 
" ball " lightning, is very rare, the appearance of 
incandescent " balls of fire " which move slowly 
along and then explode. It is probable that some 
of these recorded balls, if real, are not electrical 
phenomena, but meteoric masses from outer 
space rendered incandescent by passing through 
the atmosphere; but there seems no reason to 
doubt that this kind of discharge occasionally 
occurs, since similar phenomena have been 
accidentally produced in the discharge of 
electrical apparatus. 

" Cavallo gives an account of a fireball slowly 
creeping up the brass wire of a large highly charged 
Leyden jar and then exploding as it descended, 
and Plante has observed similar but smaller 
globular discharges from his 'rheostatic machine' 
charged by powerful secondary batteries." S. P. 
THOMPSON. 

The sound, " thunder," heard directly after the 
lightning flash, though the common parlance 
" thunder and lightning " might seem to indicate 



208 Weather Science 

the reverse, is due to the heating effect of the 
discharge. The spark heats the air in its path, 
and causes a sudden expansion (partial vacuum), 
followed by a rush of air from surrounding 
regions to fill up this partial vacuum. If the 
path be nearly straight and fairly short, one 
sharp loud clap is heard ; but if the path be 
long and broken, there will follow a succession 
of sounds rattling after one another, whilst the 
echoes from the clouds will come rolling in 
afterwards. Owing to the fact that the speed 
of light is enormously great (186,000 miles per 
second in vacuo, and only slightly less in air), 
the flash is seen almost the instant that it is 
produced, but the speed of sound in air being 
only about 1,100 feet per second, or, roughly, 
1 mile in five seconds, the reports from the 
clouds at different distances from the observer 
will be heard one after the other in order of 
their distance. By counting the interval of time 
between the flash and the sound of thunder, the 
observer may estimate his distance from the 
thunder-cloud (allowing 1 mile for every five 
seconds), and from the longer or shorter interval 
between flash and report he may ascertain 
whether the storm is receding from, or approach- 
ing towards, his station. 

The physiological, though somewhat erratic, 
effects of lightning in causing death to man and 
other animals, destruction more or less complete 
to buildings, especially such as are lofty and 



Lightning Conductors 209 

unprotected, are largely described by Flammarion 
and other writers. 

Just as we owe the discovery of the electrical 
nature of lightning to the genius of Benjamin 
Franklin, whom his own king and countrymen 
in later days ungratefully excluded as a traitor, 
though he was in reality a far greater benefactor 
to the human race than those who reviled him, 
so, too, to him is due the suggestion of lightning 
conductors. Knowing that " electricity " travels 
by preference along metal rods and other good 
conductors, he proposed to erect " upright rods 
of iron made sharp as a needle," and from the 
foot of those rods a wire down the outside of a 
building (church, house, etc.) to the ground, 
whereby "the electrical fire might be drawn 
silently out of a cloud before it came near 
enough to strike." On the principle that there 
is no free charge on the inside of a hollow 
conductor, the late Clerk Maxwell proposed to 
cover the outside of houses, etc., with a net- 
work of wires, thus shielding the interior from 
external storms. 

A good deal of discussion has arisen of recent 
years as to the best material for lightning rods 
or conductors, and the controversy between Sir 
Oliver Lodge and Sir W. Preece of the Post 
Office on this point is well known. Sir Oliver 
Lodge, holding that the lightning flash is of an 
oscillatory nature, which indeed is now well 
known to be the case with the discharge of the 



210 Weather Science 

Leyden jar, shows that the best conducting 
substance (copper) is not necessarily the best 
material for a lightning rod, and that on the 
whole iron is to be preferred, though its electrical 
" conductivity " is less than that of copper for 
continuous current. It has also the advantage 
of being cheaper. Sir Oliver Lodge has shown 
that apparently similar lightning discharges may 
take place in at least two distinct ways, for one 
of which the ordinary form of lightning con- 
ductor, as usually set up, is of no service. To 
secure full protection under all conditions, it 
has been suggested to replace the single rod 
of Franklin by a network or skeleton of rods 
or wires enclosing the building it is desired to 
defend. All parts of this network should be 
in good metallic connection with metal work, 
pipes, etc. ; metal inside and outside the build- 
ing should be connected to it, and it should be 
in good connection with the earth at as many 
points as possible. To secure that the earth 
connection is good, the earth plates should be 
at such a depth as to be in contact with water 
or moist earth all the year round (Stewart). 

" Modern views of electricity," as well as the 
singular behaviour of lightning in sometimes 
persistently avoiding "good" conductors and 
travelling where least expected, have shown 
that the whole matter is not so simple as the 
"practical man" has been hitherto inclined to 
suppose. 



Hail 2ii 

" Currents " do not flow through conductors, 
and the lightning flash, with its surgings and 
" splashes," often seems to avoid good con- 
ductors, and choose out apparently worse and 
more erratic paths. A hollow core of metal 
seems as good as a solid rod, and the phenomena 
of " self-induction " render iron, though a less 
good conductor than copper, a better guide for 
the "electric fire of Nature." 

Hail, "frozen rain," is frequently thought to 
be in some way caused by electrical action, 
though doubt has been thrown upon this con- 
nection. Certain it is that hail most commonly 
falls during thunderstorms, whether in summer 
or in winter. It has been supposed that rain- 
drops are carried upwards into the higher regions 
of the air, where they become solidified at the 
low temperature there prevailing, and fall again, 
sometimes rising once more. These alternate 
meltings and freezings taking place, the hail- 
stones sometimes increase to a great size, some- 
times over one foot in circumference. Much 
damage is done by hailstones in tropical countries, 
and sometimes also in parts of Europe, as is also 
related in chap. ix. 

During a thunderstorm the appearance of the 
sky is usually very characteristic, the expres- 
sion " thunder-cloud " being well known. The 
heavy, cumulus clouds, with admixture of cirro- 
stratus above and " cloud curtain of loose 
texture below," sometimes produce intense dark- 



212 Weather Science 

ness. The cirro-stratus clouds, when coming in 
front, may sometimes be 10 to 50 miles in 
advance of the storm. After this come the 
" thunder-heads " of cumulus clouds, and under 
this the "rain curtain," with a "squall cloud" 
below (Archibald). When the "squall cloud" 
arrives, the wind changes in direction, the air 
becomes cooler, and the barometer rises slightly. 
Then the rain or hail begins to fall, the lightning 
flashes, and the thunder rolls, until the centre 
of the storm passes. In general, thunderstorms 
are associated with great differences of tempera- 
ture in adjacent air masses, and two classes of 
storms are usually defined: (1) heat thunder- 
storms, and (2) cyclonic thunderstorms. The 
former are chiefly confined to tropical regions, 
and to the summer time in more temperate 
climates ; the latter may occur also in winter, 
are characteristic of our own Atlantic coasts, and 
are so named from their connection with cyclonic 
disturbances. They are not so dependent on 
sun heat as the other kind, frequently take place 
at night, and come along accompanied by gales 
of wind. Though not so violent as the heat 
thunderstorms, they are perhaps more dangerous, 
since the clouds being at a lower level, the 
lightning discharges more frequently strike the 
ground. 

One theory as to their occurrence is that they 
are due to the ascent of warm moist air by 
convection. In " summer storms " the cloud is 



Periodicity of Thunderstorms 213 

isolated and continuous, often from 1,000 feet 
upwards to the cirrus level of 30,000 feet. It 
spreads out in a sheet in all directions, so that a 
thunderstorm cloud of this kind often presents 
the appearance known by the name of " anvil 
shape" (Archibald). The cyclonic thunder- 
storms are supposed to be due to the cooling 
of the upper air producing similar effects to 
the heating of the warmer air, in disturbing 
electrical equilibrium. 

There is both an annual and a diurnal 
periodicity in the frequence of thunderstorms. 
More storms occur in summer than in winter, 
and the " summer storms " are most frequent in 
the early afternoon, when the temperature is at 
its highest for the day. The winter storms of 
the " cyclonic type," on the other hand, though 
less frequent than the "heat" thunderstorms, 
occur at all hours, frequently during the night ; 
they are more frequent on the Atlantic seaboard 
(western coasts) than on the other, because 
owing to the proximity of the sea, great contrasts 
of temperature at different heights in the atmos- 
phere less frequently arise. Nevertheless, that 
storms are associated with sudden changes of 
temperature is evident from the fact that hail 
so often falls during their occurrence, and that 
the thunder-clouds are mainly of the cumulus 
type, whose presence indicates the existence of a 
colder and drier layer of air immediately above 
that in which the clouds form ; and thirdly, 



214 Weather Science 

changes of wind occur during their continuance, 
occasioning a rapid fall of the thermometer. 

"The way in which changes of temperature 
tend to produce electrical disturbances is not as 
yet explained, but it is undeniable that they 
form an essential condition for the generation 
of a thunderstorm." SCOTT. 

The thunderstorms in Iceland all occur in 
winter, yet it was at one time supposed that 
they were quite unknown in Arctic regions. 
On the other hand, they are of daily occurrence, 
and often of terrific grandeur, in the tropics, such 
as we in more temperate zones can form no idea 
of. Even in South Africa fearful storms are of 
frequent occurrence. In Abyssinia the number 
of four hundred and ten thunderstorms was given 
as the average annual quantity from four years' 
observations by D'Abbadie; nearly all of these 
happened in the afternoon hours. Out of one 
thousand nine hundred and nine storms recorded 
in six years, only twenty-two occurred between 
midnight and 11 A.M. (Scott). 

Of a somewhat different character from 
ordinary lightning is the luminous appearance 
known usually as "St Elmo's fire," a pheno- 
menon sometimes observed during thundery 
weather. Brushes of pale bluish light are seen 
playing at the tops of the masts of ships, tree 
tops, and other pointed objects, and the whole 
phenomenon seems of the same nature as the 
" glow " or " brush " discharge of a frictional 



St Elmo's Fire 215 

electrical machine, as indeed was noticed by 
Franklin, who, as already stated, first drew 
attention to the similarity of the phenomena 
of lightning and "frictional electricity." In 
ancient times this appearance seems to have been 
considered an omen of good fortune for those 
by whom it was seen, and, unlike lightning, no 
damage follows from its manifestation. In more 
modern times we find the son of Columbus say- 
ing that the sailors heralded the appearance of 
" St Elmo " as a sign that the fury of the storm 
was over. It is sometimes accompanied by a 
hissing noise like that made by damp powder 
when ignited. It is stated that a similar appear- 
ance has been seen at the end of weapons. The 
soldiers of Csesar's fifth legion were once alarmed 
by noticing that the ends of their lances seemed 
to be on fire, and a similar phenomenon happened 
during the campaign of Belisarius against the 
Vandals (Flammarion). 

A rarer phenomenon accompanying the dis- 
charge is a kind of electric " hum " or buzz, 
sometimes heard on mountain tops, vouched for 
by M. de Saussure. He, with several friends, 
ascended the summit of Sarley in the Grisons 
on the 22nd of June 1867. After a shower of 
sleet the members of the party had laid by their 
alpenstocks and were resting to take a meal, 
when M. de Saussure felt a pain in his back, 
like that which would be produced by driving 
a pin slowly into his flesh ! Thinking that his 



216 Weather Science 

overcoat had pins in it he took it off, but the 
painful feeling increased. On examining his 
underclothing, he found nothing to cause this 
sensation. Meanwhile, he noticed a sound which 
reminded him of the humming of bees, and at 
the same time he noticed that the sticks of 
himself and companions, which were resting on 
the rocks, gave forth a loud singing noise like 
that of a kettle when the water is on the point 
of boiling. He then understood that the painful 
feeling in his back proceeded from an electric 
flow which was going on from the summit of 
the mountain. Experiments with the sticks 
did not cause the latter to emit any light or 
luminous appearance that could be detected. 
After a short time " they all felt strong currents 
escaping from their clothes, ears, hair, and all the 
prominent parts of their bodies, as well as from 
the sticks, and hastily left the summit of the 
mountain " (Flammarion). 

The Aurora (Borealis or Australis, according 
as it is seen in the Northern or Southern 
Hemispheres respectively) is also a phenomenon 
evidently of an electrical character, and so comes 
properly under the subjects treated of in this 
chapter. 

It is an occasional phenomenon in this country, 
the " Northern Lights " in popular parlance, but 
within the Arctic Circle it is of almost nightly 
occurrence ; according to Nordenskiold the 
terrestrial globe is perpetually surrounded at 



The Aurora 2 1 7 

the Poles with a ring of light, to which he gives 
the name of the " aurora glory." As seen from 
Europe, it usually appears in the form of a 
number of streaks or streamers of a reddish 
tinge (occasionally mingled with other colours), 
which either radiate in a fan-shape form from 
the horizon nearly in the direction of the 
magnetic north, or else they form a kind of 
arch in the sky whose head is northwards. 
The whole generally has a characteristic, un- 
steady, and vibrating appearance, the streamers 
flickering and varying both in colour and bright- 
ness, being usually very faint, but occasionally 
of considerable brilliancy. The appearance of 
a brilliant aurora is usually accompanied by 
irregular disturbances of the magnetic compass 
needle all over the world ; in other words, 
what is called a " magnetic storm." The needle, 
apart from its general north and south direction, 
and the slow daily oscillation backwards and 
forwards from a mean position, at times moves 
in a much more irregular manner. 

These movements, though very small in 
amount, are known by the name of " magnetic 
storms," and are often sufficient to interfere with 
the regularity of telegraphic signalling. They 
have some as yet unknown connection with out- 
bursts on the sun, the appearance of large sun- 
spots and unusually bright prominences occurring 
simultaneously with auroras and magnetic storms 
on the earth. This has been strikingly shown 



2i8 Weather Science 

on various occasions. On the afternoon of 
1st September 1859, Messrs Carrington and 
Hodgson, observing the sun simultaneously, 
saw two luminous objects make their appear- 
ance on the disc at the edge of a great sun-spot, 
of a brightness, at least, five or six times that 
of the neighbouring regions of the solar surface 
" photosphere." These objects moved over about 
36,000 miles in five minutes, and then dis- 
appeared. A great magnetic storm and brilliant 
aurora followed on the same night. On 3rd 
August 1872 the late Dr Young observed that 
the chromosphere 1 in the neighbourhood of a sun- 
spot was greatly disturbed, and jets of luminous 
matter of intense brilliance were projected 
upwards. 

Magnetic storms were recorded at Greenwich 
and Stonyhurst, as nearly as possible at the 
same moment that these disturbances were seen 
in America. In September 1909 a very large 
composite sun-spot, which underwent frequent 
and striking series of changes, crossed the 
central meridian of the sun's disc on the 23rd, 
directly after which " there broke out the most 
intense magnetic storm experienced for more 
than a third of a century." At night, auroras 
of great brilliance were seen, both in Europe 
and in the Southern Hemisphere. 

1 The chromosphere is a layer surrounding the sun's disc, visible 
only by the help of the spectroscope, in which " prominences " 
appear from time to time. It is so named from its usually coloured 
(red, etc.) appearance. 



Magnetic Storms 219 

It is thus fairly evident that there is some 
intimate connection between solar outbursts, 
magnetic storms, and terrestrial auroras, though 
it does not follow that the former in any sense 
"cause" the latter. 

It may perhaps be as well to state that neither 
a magnetic storm nor an aurora is accompanied 
by any simultaneous development of atmospheric 
electricity or thunderstorm, properly so called. 
The term "magnetic storm" may perhaps 
appear somewhat of a misnomer, for the most 
violent movements of the needle are, after all, 
only very minute, never exceeding a few 
minutes of arc to the right or left of its mean 
(undisturbed) position. The ordinary move- 
ment of the compass needle in this country 
exhibits a daily and annual variation somewhat 
as follows: 

About 7 A.M. the needle moves towards the 
west till about 1 P.M., and attains a "westerly 
declination" of about 8' to 10' from its mean 
position, then it slowly travels back eastwards 
till about 10 P.M. ; it then usually remains quiet, 
or nearly quiet, till the following morning, 
but in summer time it sometimes moves again 
slightly westwards and back again during the 
early morning. These changes appear to be 
in some way connected with the sun's position, 
and perhaps the moon, too, may also be con- 
cerned in these movements, which are smaller 
in winter than in summer. There is also an 



22O Weather Science 

annual variation both in decimation and intensity, 
the " total force " being greatest in summer and 
least in winter, whilst the angle of dip, or the 
angle which a freely suspended magnetised needle 
makes with the horizon, is subject to small 
seasonable changes, apart from the progressive 
changes in these elements. 

Though perhaps somewhat outside our subject, 
we may just mention these latter phenomena. 
The magnetic needle, though pointing very 
nearly north and south, does not exactly do so, 
but at present its mean position makes an angle 
of about 15 (in London, 1910) with the true 
geographical meridian (north and south line), 
which deviation is called the magnetic declina- 
tion. This declination is not the same every- 
where, or for all time, but varies both in time 
and place. It is greater for places to the west- 
ward of England and Ireland than it is for 
London and the eastern counties, and the 
difference in angle is about 6 for the limits of 
the British islands. One end of the needle 
points (roughly) northwards, the other end 
southwards; these are called the "north-seek- 
ing" and " south - seeking " poles respectively. 
In London, at present, the north-seeking end 
makes an angle of about 15 to the westward 
with the true meridian; in Galway this angle 
is about 20 W. This deviation of the magnetic 
meridian from the true north and south was 
known to the Chinese in very early times, for 



Changes in Magnetic Declination 221 

their authors mention magnetic carriages show- 
ing " direction towards the south " ; but the 
discovery, so far as Europeans are concerned, 
is often attributed to Columbus, who found 
that in sailing across the Atlantic he came to 
a region where the needle pointed due north 
and south. The line passing through places 
where there is no declination is called the 
"agonic line." 

In addition to this variation from place to 
place we find that at the same place the declina- 
tion undergoes a progressive change, as apart 
from the small temporary diurnal and annual 
changes and those associated with so - called 
magnetic storms. When first noticed, about 
1580, the north-seeking end of the compass 
needle pointed about 11 to the east of true 
north (at London). During the seventeenth 
century this easterly declination decreased till, in 
1657, the declination was zero, and London was 
then on an " agonic line." After this, the pro- 
gressive movement continuing in the same direc- 
tion, the north-seeking end now pointed (and 
points) westward of the true meridian, increasing 
from (in 1657) to a maximum of 24 27' in 
1816, after which it has slowly diminished at the 
rate of about 7' per annum. It has been esti- 
mated that it will again point truly north and 
south in about sixty -five years time, having 
made a complete cycle in about three hundred 
and twenty years (Thompson). Throughout 



222 Weather Science 

the world the declination is changing in a 
similar manner, though the cause is as yet 
quite unknown. The angle of dip or devia- 
tion from the vertical, shown by a freely 
suspended magnetised needle, also undergoes 
variation both from time to time and at different 
places. One end usually dips downwards, the 
other upwards, though this is scarcely noticeable 
in the ordinary form of the horizontally sus- 
pended compass needle. In our latitudes the 
north-seeking end dips downwards, and the angle 
of dip is now slightly under 67 (London in 
1910). 1 In 1576 the inclination was about 71 
50' (Thompson); this increased to 74 42' in 
1720, and then diminished again to its present 
value, and is still diminishing. At the north 
magnetic pole (near Boothia Felix, latitude 70 
N.) the north-seeking end of the needle points 
straight downwards, and the dip is consequently 
90 ; at the magnetic equator the needle is 
horizontal and the dip zero. Lines joining 
places where the dip is the same are called 
isoclinic lines, roughly corresponding to the lines 
of latitude on a geographical map, whilst the 
isogonic lines still more roughly correspond to 
meridians of longitude. The " aclinic " line (line 
of no dip) may be called the magnetic equator, 
whilst the points where the north- or south-seek- 1 
ing ends respectively dip vertically downwards 

1 The mean declination at Greenwich for 1909 is given as 15 
47' W. Mean dip, 66 53' 57". "Astronomer Royal's Report to 
Board of Visitors, 18th June 1910." 



Magnetic and True Bearings 223 

(angle of dip 90 N. or 90 S.) may be (and are) 
called the magnetic poles. The third element 
(after declination and inclination), magnetic in- 
tensity or intensity of the force, whatever it 
may be, that causes the needle to set itself as it 
does, is also subject to variations, corresponding 
with those of the two other elements, both in 
its amount from time to time and in different 
places, but need not be further dwelt upon 
here. 

One caution may perhaps not be out of place 
in connection with meteorological observations 
in reference to observations of wind direction, 
the position of weather vanes, etc. Many 
persons take their directions from a compass, 
in ignorance of the fact of magnetic declination 
we have just mentioned, the deviation between 
true and magnetic north and south. In London 
and eastern England this difference amounts to 
about 15, and is yearly diminishing, but in 
Ireland it may amount to over 20 (or nearly 
two points of the compass), and it was greater 
in this country also during the last century. 

Thus a vane set up by the help of a compass, 
no allowance being made for this variation or 
decimation, will have its north point really turned 
towards N.N.W., its east point will really be 
15 to 20 to the north, and so on. The compass 
card being divided into thirty-two points, and 
there being 360 in the complete circumference, 
one point will be 3 ^ = llj and two points 22 J% 



224 Weather Science 

between which values the declination will lie 
for these islands, this being, as already stated, 
least for eastern England and greatest for 
western Ireland, increasing in a roughly westerly 
direction, the isogonic lines running N.N.E. and 
S.S.W. An error of two points may thus arise 
in giving records of wind direction, quite apart 
from any errors arising from the sluggish move- 
ments of weather vanes. 

[Note on the Aurora. Examined by the help of the spectroscope, 
the light of the aurora is shown to be due to gaseous matter , but 
all the lines of the bright line spectrum are not yet identified, one 
or two being not yet recognised as due to any known substance, 
though similar in character to those given by the electric discharge 
in highly rarefied air. This was indeed the explanation of it given 
by Franklin long before the days of spectrum analysis, and still 
seems the most generally accepted theory. The cold air near the 
Poles and currents of warmer air and vapour coming from equatorial 
regions certainly must differ in electric potential, and discharges 
will take place between them. Another theory supposes the aurora 
to be due to differences of potential set up in the upper regions 
of the air by the inductive action of the earth rotating within a 
less rapidly rotating shell of outer atmosphere ! The height of the 
auroral display above the earth is found by Dr Paulsen to range 
between 61 and 67 kilometres (about 40 miles), though apparently 
the fiery trains never seem to reach the surface of the earth, and 
they have been observed lower in elevation the further north they 
have been met with (Inwards). 

An artificial aurora was produced by Professor Lemstrom, of 
Helsingfors. He erected 011 a mountain in Lapland a network 
of wires and rods which presented many points to the sky. He 
was thus able to send beams of electricity into the air, and 
observed columns of light ascending from his apparatus more than 
100 yards into the air. By insulating his apparatus and connecting 
it by a wire with a galvanometer at the bottom of the mountain, he 
was able to observe actual currents of "electricity'' when the 
" auroral beam" rose above the mountain.] 



Zones of Temperature 225 



CHAPTER XII 

SEASONAL VARIATIONS OF WEATHER RECURRENT TYPES BUOHAN's HOT 

AND COLD PERIODS INDIAN SUMMER ST LUKE ? S SUMMER ST 

MARTIN'S SUMMER MAY AND NOVEMBER COLD SPELLS, ETC. 
SOUTHERLY, NORTHERLY, WESTERLY, AND EASTERLY TYPES (ABER- 
CROMBY) SOUTHERLY TYPE WEATHER CONDITIONS WESTERLY 

TYPE NORTHERLY TYPE EASTERLY TYPE DISTRIBUTION OF 

PRESSURE OVER THE GLOBE POLAR CYCLONES THE DOLDRUMS. 

THE heat of summer and the cold of winter, 
accompanied by a greater amount of fine 
weather in the former, and more rain and 
unsettled conditions during the latter period, 
constitute what we may call the normal or 
expected condition of affairs, though in our 
own country local and temporary irregularities 
draw so much attention to themselves that 
many persons will be inclined to think that 
the exceptions are more frequent than the 
rule. Over the whole earth, however, we 
may distinguish three characteristic regions : 
(1) the equatorial belt of calms, the "doldrums" 
region; (2) the tropical belt, "region of trade 
winds " ; (3) the " Temperate and Arctic Zones." 
The equatorial belt forms a kind of hollow 
between the two trade wind regions on either 
side of it. North and south of this on either 



226 Weather Science 

side we have the trade wind areas, where for 
months at a time the same wind blows con- 
tinuously. In the Northern Hemisphere we 
have the north-east trade wind, caused by air 
blowing from the Poles towards the Equator 
and deflected by the effect of the earth's rota- 
tion towards the right, and a south - western 
" anti - trade " wind coming from the Equator 
towards the Poles. These winds are less marked 
over the continents than over the ocean, owing 
to the influence of the former in modifying their 
direction and disturbing their regularity ; but 
in the Southern Hemisphere such winds are so 
well marked and strong that the zone of latitude 
in which they are principally felt is known as 
"the roaring forties " (40 S.). The influence of 
the ocean in modifying climates, by means of 
the currents of warm and of cold water it 
conveys from place to place, by the latent heat 
of evaporation and condensation of its water, 
raised into the air in enormous quantity through 
the action of the sun's heat, is perhaps the 
most important of any agency in altering what 
one might perhaps call the astronomical or 
latitudinal conditions, i.e., such as might be 
supposed to exist with no ocean or atmospheric 
circulation. 

In our own country, and throughout North- 
Western Europe, in addition to the general 
course of the seasons, from the heat of summer 
to the cold of winter, we have more or less 



Hot and Cold Spells 227 

recurrent periods of from three days to one 
week in length, during which periods year after 
year we have similar weather. A number of 
these periods have been discovered by meteor- 
ologists, and six cold and three hot periods are 
known by the name of Buchan's hot or cold 
spells respectively. 

The " cold days " of May, the Lammas flood 
period, St Luke's and St Martin's summers 
respectively, are well known. The following 
short account of the chief of these periods, whose 
regular return seems well established, is chiefly 
given on the authority of Mr Abercromby 
("Weather"). 

From 7th to 10th February is a spell of 
cold weather Buchan's first period. The 
weather of this period is associated with what 
is called the " northerly type," or pressure 
high over Greenland and the Arctic portion 
of the Atlantic Ocean, and low over most of 
Europe. 

The proverbial east winds of March, when 
they occur, are also usually due to the northerly 
type of weather, but there is little or no evidence 
for the supposed prevalence of equinoctial gales 
about the 21st. Indeed, neither at the spring 
nor the autumn equinox are the winds in this 
country, as a rule, of any exceptional violence, 
so that it seems somewhat extraordinary how so 
widespread an idea can have arisen. 

llth to I4ith April. Buchan's second cold 



228 Weather Science 

spell, the " borrowing days," supposed borrowed 
by March from April : 

" March borrows of April, 
Three days and they are ill." 

The difference of dates between the " old " and 
" new " style (Julian and Gregorian calendars) 
made llth April on the latter follow 31st March 
on the former. 

9th to 14>th May. Buchan's third cold period, 
the famous " three " (or six) cold days of May. 
All over Europe this periodical diminution of 
temperature occurs, due to the setting in of a 
spell of the easterly or northerly type. Fanciful 
theories as to the impact of a stream of meteors 
with the earth which cut off some of the sun's 
light and heat, and ideas as to the periodic 
passage of our planet through an extra cold 
region of space, have been promulgated, but such 
an occurrence would lower the temperature not 
only over Europe, but over the rest of the world 
also, which does not appear to be the case. 

June. A cold spell in the second or third week, 
accompanied by weather of the northern type. 
Eredia, in a recent paper to the Accademia 
Lincei, has clearly established the existence of 
this period of low temperature during the second 
decade of June throughout Italy, more pro- 
nounced in Lombardy and Venetia and the 
interior of Central Italy than in the coast 
regions. During this period there is usually a 



St Swithin's Day 229 

region of low pressure in Russia, and one of high 
pressure over Spain, so that the weather is of 
the "northerly" type. 

29th June to 4tth July. Buchan's fourth cold 
period. 

12th to 15th July. Buchan's first warm period- 
July 15th is " St Swithin's Day," the popular 
legend as to the wetness or otherwise, " St 
Swithin is christening the apples," is well known, 
though resting on a very slender foundation. 
The occasional persistence of the weather current 
during this time for a fortnight or so, whether 
wet or fine, is an undoubted fact, but it neverthe- 
less sometimes happens that a dry St Swithin's 
Day is followed by a good many wet ones, and 
vice versa. What may have happened once is 
no promise that such a thing will occur again. 

2nd to 8th August. A wet period, the 
"Lammas floods" of Scotland (Lammas Day, 
1st August). 

6th to llth AuguM. Buchan's fifth cold period. 

12th to 15th August. Buchan's second hot 
period. 

2Uh August (St Bartholomew's Day). "If it 
rains this day it will rain the forty days after." 
Thus St Bartholomew is a rival to St Swithin ! 

September. Towards the end of the month, 
the " Indian summer " of North America is 
experienced ; a fine period, lasting for three or 
four days. 

ISth October (St Luke's day). A fine, quiet 



230 Weather Science 

period about this time is consequently known 
as St Luke's summer. 

6th to 12th November. Buchan's sixth cold 
period, associated with weather of the northerly 
type. 

llth November (St Martin's Day). A fine, 
warm period of a few days about this time is 
known as " St Martin's little summer," both in 
this country and in more southern latitudes. 
"Expect St Martin's summer" (Shakespeare, 
Henry F7., pt. 1, act 1, sc. 2). 

3rd to 9th December. Buchan's third warm 
period. 

The general explanation of the recurrence of 
these cold and warm periods is to be found in 
the tendency to the recurrence of certain types 
of pressure distribution at or about the dates 
given above. Weather of the northerly or 
easterly type gives rise to the cold " spells " ; 
weather of the southerly type in winter is 
attended by a warm period. Of the foregoing 
periods the cold spells of May and November 
are very regular in their recurrence from year 
to year, though not always exactly in the same 
weeks of the months; more often than other- 
wise "the cold period comes in the third week 
of each month" (Chambers). It is a popular 
superstition that a late Easter is accompanied 
by severe weather in this country, but we need 
hardly say that though instances of cold Easters 
are common enough, the evidence of statistics 



Four Types of Weather 231 

lends no countenance to this idea. Three warm 
periods during the autumn, the first about the 
end of September, the second about 18th October, 
and the third early in November, as already 
mentioned, are known popularly by the name of 
" summers " Indian summer, St Luke's summer, 
and St Martin's summer respectively and fanci- 
ful theories as to their cause and recurrence have 
been propounded, some attributing their warmth 
to the latent heat liberated during the condensa- 
tion of vapour, or the freezing of water to form 
ice, etc. 

Four principal types of weather, corresponding 
to four forms of pressure distribution, have been 
recognised by Mr Abercromby as occurring in 
these islands and throughout Western Europe 
generally, to which he has given the convenient 
names, from the prevailing wind in each, of 
southerly, northerly, westerly, and easterly 
respectively. In the first or southerly type the 
Atlantic anticyclone, so prevalent a feature of 
the Central Atlantic, whose range south and 
west is fairly constant, but which varies con- 
siderably in its northern limits, lies to the east 
or south-east of the British Isles, whilst to the 
north of it is a region of low pressure, in which 
cyclones are continually generated and come in, 
encountering a region of higher pressure over 
the continent, and either die out or pass towards 
the north-east. At times the low-pressure area, 
ordinarily over the North Atlantic, spreads into 



232 Weather Science 

these islands and displaces the high - pressure 
region eastwards ; during this state of affairs 
we have fine weather and little wind, with a 
low barometer. This type of weather is often 
prevalent during mild winters. 

The broad sequence of weather conditions 
during the prevalence of this type is, roughly, as 
follows: The barometer falls, the temperature 
rises, there is a gloomy sky, and drizzling rain 
sets in. The wind, usually southerly or south- 
westerly, backs slightly and increases in force. 
After a time the barometer ceases to fall and 
begins to rise again, the wind gradually falls, 
temperature decreases, and the sky becomes 
clearer. On the following day a similar 
sequence occurs, and this alternation sometimes 
lasts for days or even weeks at a time. When- 
ever a persistent spell of this type is seen to 
have set in, the work of the forecaster is fairly 
easy, for he has only to recur to his experience 
of former similar spells. The cyclones coming 
in from the Atlantic do not pass over, so that 
our country will only be under the influence 
of their fronts. Increases of intensity, rising 
in force to a gale or storm, may be expected 
from time to time, but the general character of 
the weather and direction of the wind remain 
without much change, till this type merges 
insensibly into the western on one side, or 
rather more abruptly into the eastern on the 
other side. 



Westerly and Northerly Types 233 

The westerly type of weather is characterised 
by the development of small cyclones on the 
north side of the great Atlantic anticyclone. 
These move quickly eastwards, and often die 
out after they have become detached from 
the latter. They vary greatly in intensity, 
and at times, when their centres cross these 
islands, destructive storms, especially occurring 
in spring and autumn, are experienced in their 
passage. Most commonly, however, their paths 
are further to the north, and the wind merely 
backs a little first from the south-west, veering 
towards the west afterwards, and is of moderate 
intensity, accompanied by some rain. Some- 
times when the Atlantic anticyclone extends 
to the southern part of our islands, the north 
of Europe is covered by small cyclones, no 
rain is developed, but only cloud, with fine dry 
weather. This type is very common, perhaps 
the most frequent of all in our latitudes. 

The general character of the temperature is 
about the average for the season, but in winter 
the influence of the wind prevents much frost, 
whilst in summer the temperature may be below 
the average if much cloud is present. 

The northerly type is characterised by the 
presence of a large anticyclone over Greenland 
and the extreme North Atlantic. The latter 
sometimes unites with the Atlantic anticyclone 
so often referred to, or is only separated from 
it by a col. To the east of this, lying over the 



234 Weather Science 

greater part of Northern Europe and Russia, 
there is a large area of low pressure, whence 
issue cyclones and secondaries. This type is 
exactly the converse of the southerly, the latter 
type presenting cyclone fronts and southerly 
winds to Europe, the former northerly winds 
and rear portions of cyclones. During the 
prevalence of this northerly type the sequence 
of weather is, roughly, as follows : The baro- 
meter falls, the wind changes towards north- 
east with cloudy sky, then some rain falls, with 
increase of temperature. After a while the 
sky clears up, the wind backing through north 
towards north-west, the barometer rises, and the 
temperature falls. The sky throughout has a 
characteristic "hard" appearance, the general 
temperature will be below the average for the 
season, and the air will be in general dry from 
the prevalence of northerly winds. 

The fourth type is that known as the easterly 
one. Here there is a fairly persistent anti- 
cyclone over Scandinavia, whilst the Atlantic 
anticyclone does not extend so far north as 
usual, but there is a col formed between the 
two which, crossing Europe, profoundly affects 
the course of the weather changes. Then the 
cyclones coming from the Atlantic either pass 
through this col in a south-easterly direction, or 
else they are arrested and remain over Western 
France and the Bay of Biscay. Occasionally 
cyclones forming on the southern edge of the 



Easterly Type 235 

Scandinavian anticyclone have their centres over 
Southern Europe, and move westward, contrary 
to the almost general rule for cyclone motion. 
Thus, while for cyclones formed in the three 
previous types of pressure distribution the move- 
ment is always easterly, for the easterly type 
we may sometimes get westerly progression. 
During the persistence of this type the general 
character of the biting east wind is characteristic 
and well known. The temperature is generally 
low, the wind always from some point of east, 
occasionally veering towards south-east, or back- 
ing towards the north - east. In our islands 
this type of weather sometimes persists for two 
or three weeks consecutively, and gives rise to 
destructive storms. It is stated that nearly one 
half of the wrecks on the British coast are due 
to gales of this class (Abercromby). 

These four types of pressure distribution 
describe most of the weather successions in 
Western Europe, including our own islands, and 
by reference to them considerable simplification 
of the "ceaseless and complicated changes" is 
introduced, though of course the simplicity of 
conditions prevailing in more tropical regions 
is still far off. The recurrence of certain kinds 
of weather at about the same time each year 
is to be referred to this tendency of types to 
repeat at definite times, whilst the changes of 
type are often fairly regular in character. Thus 
the southerly type may change gradually into 



236 Weather Science 

either the westerly or the easterly, but not in 
any case into the northerly, the westerly type 
may become southerly or northerly, but not 
jump into easterly, each type changing only 
into its next neighbour on either side, never 
into its opposite (as east into west, north into 
south) ; for whilst a slight shift of pressure may 
modify one type into the next, a change to the 
opposite type would involve a total rearrange- 
ment of pressure over the whole Northern Hemi- 
sphere (Abercromby). 

The most general description of the distribu- 
tion of pressure throughout the globe is to 
roughly divide this into three zones : (1) There 
is an equatorial zone of low pressure ; (2) a 
tropical belt of high pressure ; (3) a temperate 
and arctic zone of generally low pressure over 
which occasionally areas of high pressure appear 
for a considerable time. On the theory of Ferrel, 
the general circulation of the air may be con- 
sidered as consisting of two huge cyclones, one 
round each pole, in which the air rotates in 
the same direction as that of the earth itself. 
On the equatorial side of each of these there is 
a belt in which the air rotates in the contrary 
direction. (These are the " trade-wind " belts.) 
Between these the air is heaped up into two 
regions of higher pressure, whose highest mean 
values occur at about latitudes 35 N. and 
30 S. respectively. The equatorial belt lies 
over the Sahara in the old continent, and the 



The Doldrums 237 

Amazon valley in the new; over the Atlantic 
Ocean the region is well known to sailors as 
the "doldrums," and much dreaded by them 
in the old sailing-ship days, for their ships lay 
often for weeks on the water without motion. 

" Day after day, day after day, 
We stuck, nor breath nor motion, 
As idle as a painted ship 
Upon a painted ocean." 

COLERIDGE, Ancient Mariner. 

These are regions of " unbearable calm broken 
occasionally by violent squalls, torrential rain, 
and fearful lightning and thunder." 

The tropical belt, north of this "doldrum" 
region, comprises a region of high pressure ; 
developing into great anticyclones, whose length 
usually lies east and west. Though most of 
these are formed in different regions from time 
to time, one is very constantly found over the 
Central Atlantic, and from its important bear- 
ing on the weather of Europe and the United 
States it is, as mentioned, called the Atlantic 
anticyclone. The north-easterly and easterly 
winds on the southern side of this anticyclone 
are known as the "trade winds." Few cyclones 
are formed to the south of this Atlantic anti- 
cyclone, but its north side is the origin of the 
cyclone storms, which, moving towards the east, 
give a prevailing character to European weather. 
On the south - east side a few cyclones occa- 
sionally form, which either work round slowly 



238 Weather Science 

to the south-west, or else go eastwards ovei 
the Straits of Gibraltar. 

The temperate and arctic zone extends from 
the tropical belt to the Pole. This, though in 
its general character as mentioned above, a 
region of low pressure, is continually varying 
in this respect, and large persistent areas of 
high pressure appear. Thus changeable weather 
with variable winds characterise this region 
(which includes our own islands). 




GREENWICH OBSERVATORY (METEOROLOGICAL SECTION). 

(Reproduced by kind permission of R. Inwards, Esq., F.R.A.S., from his Presidential 
address " Observatories.") 



To face p. 239. 



Observatories 239 



CHAPTER XIII 

OBSERVATORIES FIRST ORDER, SECOND ORDER, AND THIRD ORDER 
STATIONS GREENWICH OBSERVATORY SELF-RECORDING INSTRU- 
MENTS KEW OBSERVATORY SECOND ORDER STATIONS THEIR 

WORK BI-DAILY OBSERVATIONS INSTRUMENTS OBSERVATIONS 

MORNING SERIES EVENING SERIES FORECASTS OF WEATHER 

STORM WARNINGS LIST OF DISTRICTS DAILY AND WEEKLY 

WEATHER REPORTS NOTE ON METRIC AND ' ' ENGLISH " MEASURES 

TEMPLE OF THE WINDS; ATHENS. 

THROUGHOUT our islands, in addition to the 
larger institutions under Government auspices, 
such as the Greenwich and Kew Observatories 
in the neighbourhood of London, or the 
observatories such as those at Oxford (Radcliffe), 
Stonyhurst, Cambridge, Durham, etc., supported 
and maintained by the universities or other 
educational authorities, there are a number of 
other smaller meteorological stations, where 
regular observations of the barometer, ther- 
mometer, etc., are made daily; also, in most 
of these, records of rainfall, the amount of sun- 
shine and general atmospheric conditions are 
observed. These are almost entirely supported 
by private enterprise, and much useful work 
is done, the results of which are sent in weekly 
and monthly reports to the Meteorological Office 



240 Weather Science 

and other central authorities. Some of the larger 
of these institutions send also daily telegraphic 
messages, warning of coming storms, etc. Three 
classes of observing stations are semi-officially 
recognised. 

A station of the first order is one in which 
meteorological observations are made by means 
of hourly readings, or the use of self-recording 
instruments (barometers, thermometers, rain- 
gauges, sunshine-recorders, anemometers, etc.). 

A station of the second order is one where 
complete and regular observations of the air- 
pressure, force of wind, temperature of air and 
ground, moisture, cloud, rain, sunshine, etc., are 
made, usually twice daily. 

A station of the third order is one where 
only some of these elements are observed. At 
some places there is only a rain-gauge installed, 
more than three thousand of such stations send- 
ing records of rainfall (measured daily at 9 A.M.), 
to Dr Mill, at Camden Square, London, head 
of the British Rainfall Organisation, under whose 
auspices a yearly volume, " British Rainfall," is 
published, giving the results of these (mainly 
volunteer) observations. 

The earliest systematic meteorological observa- 
tions were necessarily first made at astronomical 
observatories. All astronomical observations 
being made through the medium of the atmos- 
phere, and the conditions of visibility, refraction, 
etc., depending so closely upon the temperature, 



Greenwich Observatory 241 

pressure, and humidity of the latter, the necessity 
for accurate observation of these elements led to 
the regular reading of the barometer and ther- 
mometers by astronomers. At the Observatory 
at Paris, and the Royal Observatory, Greenwich, 
regular meteorological observations have been 
made almost from the foundation of these 
institutions ; rainfall and temperature records 
at Paris extend over two hundred years. At 
Greenwich, founded almost simultaneously with 
the sister Observatory of Paris, the records are 
almost equally complete. Standard barometers, 
thermometers sunk at different depths in the 
earth (usually four, at depths of 24, 12, 6, and 
8 feet respectively), are all regularly read at 
least once daily. 

Stevenson and Glaisher screens containing 
wet and dry bulb thermometers, maximum and 
minimum self-registering instruments, are placed 
in the grounds, the newest addition being " the 
magnetic pavilion " enclosure. The Astronomer 
Royal's report for 1910 (the last prepared by 
Sir W. Christie) states that " registration of 
atmospheric pressure, temperature of the air and 
of evaporation, pressure and velocity of the wind, 
rainfall, sunshine, and atmospheric electricity, has 
been continuously maintained. Cloud observa- 
tions, in connection with the international balloon 
ascents, have been made with a Fineman 
nephoscope." In a shed is placed the apparatus 
for photographic registration of the dry and 



242 Weather Science 

wet bulb thermometers. The indications of 
these instruments are recorded by means of the 
following arrangement. 

A sheet of sensitised paper is rolled round an 
upright drum, which is moved slowly round by 
clockwork, whilst a beam of light passing to a 
greater or less degree through the thermometer 
tubes, according to the height of the mercury 
column, after falling upon the paper, produces 
two traces upon the latter, each trace being 
bounded on one side by a wavy line, and thus 
automatically and continuously records the varia- 
tions in temperature of the two instruments. 
The standard barometer is read directly at 
frequent intervals, and a continuous record of 
the variations of pressure is obtained by photo- 
graphy, in a similar manner to that in which 
the variations of temperature are recorded ; light 
falling upon sensitised paper, being stopped to 
a greater or less degree, according as the height 
of the mercury column varies from time to time. 
A sunshine recorder, of the Campbell Stokes 
pattern, is placed on the top of the magnetic 
building, and records of sunshine have been 
obtained in this way since 1876. Various rain 
gauges, at different heights above the level of 
the ground, and self-recording rain gauges, are 
also in regular use. The self-recording rain 
gauge makes its record in the following manner. 
The rain falls into a vessel which is suspended 
by spiral springs. This vessel falls as it becomes 




KEW OBSERVATORY (NATIONAL PHYSICAL LABORATORY). 
(Reproduced by kind permission of R. Inwards, Esq., F.R.A.S.) 



To face p. 343. 



Kew Observatory 243 

heavier, with more or less rapidity, according 
to the rate of rainfall. By means of a cord 
passing from the vessel, over a pulley to a pencil, 
the rate of fall is registered on a sheet of paper 
which is moved by a clockwork mechanism. 
When 0*25 of an inch has been collected, the 
vessel automatically empties itself, the pencil 
goes back to zero, and the record begins again. 

At the Kew Observatory, Richmond, in 
the Old Deer Park, originally the private 
Observatory of King George III., and now a 
branch of the National Physical Laboratory 
(Observatory Department), in addition to im- 
portant work in the testing of instruments, 
thermometers, barometers, chronometers, etc., 
for which the " Kew Observatory " certificates 
are desired, there have been going on for a long 
time continuous meteorological and magnetic 
observations, both with self-recording instru- 
ments, and by means of regular eye readings. 
The observations carried on at Kew, though to a 
great extent similar to those at Greenwich, have 
an independent value of their own, and whilst 
the testing of chronometers and watches forms 
a large part of the work at Greenwich, the staff 
at Kew, in addition to their regular scientific 
work, are more occupied with the testing of 
meteorological and magnetic instruments, and 
in the training of observers, who propose to 
work in remote regions. Here, as also at the 
Jermyn Street Museum, Piccadilly, is to be 



244 Weather Science 

found a glycerine barometer, which, owing to 
the smaller density of glycerine as compared 
with mercury, has a much longer tube, so that 
slight variations in pressure give rise to greater 
movements of the glycerine column (in inverse 
proportion to the relative density of the two 
liquids). The specific gravity of glycerine being 
only 1*27, whilst that of mercury is 13'6, the 
former column will be more than ten times as 
long as the latter. Glycerine has the dis- 
advantage, however, of solidifying at a higher 
temperature, and cannot easily be obtained as 
pure as the liquid metal. 

Amongst other observatories, where systematic 
meteorological observations are carried out on a 
large scale, may be especially mentioned the 
great Observatory at Pawlowsk, in Russia, fitted 
with the most complete series of meteorological 
instruments in the world. A very detailed 
description of this institution has been given 
by Dr Waldo, in his well-known book, " Modern 
Meteorology." Of special interest from the 
nature of the observations of cloud phenomena 
and other features of the upper air are the 
mountain observatories, most of which are 
situated in the Alpine regions of Europe. In 
Switzerland, the Santis and Rigi stations ; in 
Austria, the Sonnblick, the highest in Europe, 
10,000 feet above sea - level ; in Italy, the 
Observatory of Etna; in France, the Puy de 
D6me ; the observatory at the top of the Eiffel 



Second Order Stations 245 

Tower, Paris; and the observatory in America, at 
Mount Hamilton, California (especially famous 
for the Lick telescope and the wonderful series 
of astronomical observations made there), are 
amongst the best known of these stations. For 
some years a meteorological observatory was 
maintained on Ben Nevis, in Scotland, the 
highest of the British mountains ; supported by 
a grant from the Government and the Royal 
Meteorological Society, but it has been since 
abandoned. The highest meteorological station 
in the world is probably that of El Misti, in 
Peru, containing a number of self-recording 
instruments, at the height of 19,000 feet above 
the sea, erected under the auspices of Harvard 
College (U.S.A.). 

At most private astronomical observatories 
in this country there is to be found the 
installations of a second order station where 
observations are regularly carried on, whilst 
in a few (e.g., Stonyhurst, Parsonstown, etc.) 
the hourly readings and self-registering instru- 
ments entitle these to be called rather first 
order stations. The regular instruments for a 
second order station are the following: There 
must be a standard barometer (preferably of 
the Fortin pattern with movable cistern), with 
vernier and attached thermometer. Next there 
should be a pair of thermometers, wet and 
dry bulb respectively, whose readings should be 
regularly checked by comparison with standard 



246 Weather Science 

instruments or in other well-known ways ; also 
a maximum and a minimum thermometer. 
These instruments should be placed in a posi- 
tion where they are freely exposed to the air, 
but sheltered from sun, rain, and excessive wind. 
For this purpose they are usually placed in 
some kind of screen, of which Stevenson's well- 
known form seems to be on the whole the 
most generally suitable. The wet and dry 
bulb thermometers should be fastened to the 
wall of the screen, side by side, in an upright 
position, their bulbs being a little above the 
floor of the screen, whilst the other two ther- 
mometers should hang in a horizontal position 
within the screen, supported by hooks which 
fit into nails from the wall, so that they may 
be "set" after each reading. These instru- 
ments are usually at a height of about 4 feet 
from the ground. In addition there may be a 
black bulb thermometer for measuring " solar 
radiation," supported in a horizontal position in 
the open air on a kind of stand at about the 
same height as the other thermometers, and 
lastly a grass minimum thermometer, which is 
suspended on two wooden Y's at a height of 
1 or 2 inches above the ground. 

Next after the barometer and thermometers 
comes the rain gauge or gauges. These should 
be set up in a well-exposed position, and not 
near trees, walls, or other buildings. The height 
of the rim of the "ground" gauge should in 



Sunshine Recorder 247 

general be about 1 foot. Upper gauges are 
sometimes placed on a roof or wall, and if 
compared with one upon the ground, it will 
be found that the amount collected will be 
less in the former case than in the latter. 

For instance, at Markree, in Ireland, it was 
found that of two rain gauges, one 16^ feet 
above the ground and the other 6 inches from 
the surface, the amount registered by the former 
had to be multiplied by 1-2045 to reduce it 
to the level of the lower gauge. A total annual 
rainfall was 44*87 inches as recorded by the 
lower gauge, but only 37*25 inches in the upper 
one. This difference is probably due to local 
eddies caused by wall or roof, as the case may 
be, blowing some of the rain over the upper 
gauge instead of into it. 

At most second order stations there is to 
be found a sunshine recorder, usually of the 
Campbell- Stokes' pattern, consisting of a glass 
globe which focusses the sun's rays upon a strip 
of cardboard graduated to correspond to the 
times of day when the solar image falls upon 
any point of it. 

Three series of grooves and corresponding 
cards are in use, for summer, spring and autumn, 
and winter respectively, on account of the 
different altitude of the sun in the different 
seasons. The sunshine recorder has the dis- 
advantage of not registering faint sunshine ; 
when light clouds or haze are present between 



248 Weather Science 

the sun and the instrument, the image is not 
strong enough to burn a trace on the paper. 
At some stations, however, the Jordan form 
of sunshine recorder is used instead. A trace 
due to the chemical action of the sunlight, of 
greater or less intensity, is made upon a sheet 
of sensitised paper placed in a dark chamber, 
into which light from the sun enters through 
two small apertures. 

Records of the state and amount of cloudiness, 
force of wind, etc., are usually made by eye 
estimates without specific instruments, though 
a nephoscope is in use at larger institutions, 
and various forms of anemometers of greater or 
less degrees of reliability are to be met with. 

The observations at stations of the second 
order should be made at least twice a day, and 
the hours of 9 A.M. and 9 P.M. (local time) have 
been chosen almost invariably by the volunteer 
observers throughout the United Kingdom. 
Other combinations, however, have been decided 
to be admissible, e.g., 8 A.M., 2 P.M., and 8 P.M., 
but in general the combination of 9 A.M. and 
9 P.M. is to be preferred, and it is found that 
"the means of these observations do not differ 
much from the true daily mean." At the 
morning observation hour (9 A.M.) the barometer, 
with its attached thermometer, is to be first 
read ; the latter instrument first, as the warmth 
of the observer's body will cause a slight rise 
in its temperature. 



Morning Observations 249 

In reading the barometer (if it is of the 
Fortin pattern, as is usually the case), the 
observer should be careful that the ivory point 
of the scale does not dip into the mercury, 
but just touches it, and the vernier should be 
set so that its lower edge just touches the 
convex surface of the mercury in the tube, as 
seen by the eye on a level with the front and 
back edge of the vernier and top of the mercury. 
It may be sometimes necessary to slightly tap 
the instrument to allow the " force of capillarity 
to exert its normal action " and prevent the 
mercury from sticking to the glass, and it will 
be found useful to place a piece of white paper 
behind the tube, so as to allow of setting the 
vernier accurately. Of course the instrument 
must hang vertically, and care should be taken 
to prevent the introduction of air bubbles, whose 
presence not only depresses the mercury column, 
but they also act chemically upon the liquid 
and corrode it. Next to the barometer the dry 
and wet bulb thermometers should be carefully 
read, and the water in the vessel, keeping the 
wet bulb damp, replenished if need be. These 
instruments are usually placed in a screen, 
together with the maximum and minimum 
thermometers, but these latter are not read 
till the evening observation. Distilled or rain 
water should always be used for the wet bulb, 
and the piece of muslin or worsted used to 
keep the latter damp should be changed every 



2 5 Weather Science 

two or three weeks, or more often if exposed 
to a dusty atmosphere. 

After this the rain gauge should be examined, 
and its contents poured into a graduated (glass) 
measure. It is the practice to read this instru- 
ment once a day, at the morning observation, 
and to enter the amount as having fallen on 
the previous day, inasmuch as in most circum- 
stances more rain is likely to have fallen in 
the fifteen hours from 9 A.M. to midnight of 
the previous day, than in the nine hours from 
midnight to 9 A.M. A better practice would 
seem to be to read the rain gauge at 9 P.M., 
when there would be twenty-one hours of one 
day, and only three hours of the preceding 
day, but this is not done for reasons with which 
I am not clearly acquainted. However, at 
present the rain gauge is read at 9 A.M. 

The direction of the wind, as given by the 
vane, is next to be recorded, as well as its 
estimated force on the Beaufort scale, an account 
of which has been given in a previous chapter. 
Direction should, of course, be always given 
according to true geographical and not com- 
pass bearings, though now that the magnetic 
north through variation is becoming every year 
nearer the true north (in this country) the error 
is a decreasing amount. Various forms of ane- 
mometers, of which the most frequently used 
is Robinson's, are to be met with in different 
observatories. Lind's and Dines* anemometers are 



Wind Estimation 251 

also sometimes used, the latter, like Robinson's, 
measuring the wind velocity approximately in 
miles per hour. The theory of the Robinson 
anemometer was given by the inventor in 1850 
( Transactions of the Royal Irish Academy), and 
he considered that the velocity of the centre 
of the four cups (four hemispherical cups of 
thin metal attached to the ends of two light 
rods crossing each at right angles form the 
essential features of the instrument. They 
are fastened to a vertical axis, and revolve 
under the influence of the wind, a registering 
apparatus giving the number of revolutions) 
must be multiplied by three to get the velocity 
of the wind ; but later experiments and investi- 
gations by Stowe, Dines, and others, have shown 
that this factor is too large, and that the true 
multiplier should be from about 2*0 to 2*2 instead 
of 3 for moderate speeds. But the whole " theory 
and practice" of the anemometer is still very 
imperfect, and little reliance can be placed on 
estimates of the exact number of miles per hour 
registered for wind velocities in storms and 
gales. In recording the wind it is necessary to 
be sure that the true direction of the main wind 
passing over the station be entered, and not a 
mere local eddy. The amount and character of 
the clouds are to be registered for each observa- 
tion. A clear blue sky is regarded as 0, and 
a completely overcast one is registered as 10, the 
observer estimating as carefully as possible the 



252 Weather Science 

proportions of clear and cloudy sky. Lastly, 
the general character of the weather, since the 
last observation (of the previous evening) should 
be recorded. 

The evening observations, like the morning 
ones, should commence with the reading of 
the barometer and its attached thermometer, 
which should be done as quickly, yet carefully, 
as possible. Artificial light will ordinarily be 
required for the evening observations, except at 
stations in the north of Scotland during summer, 
and an ordinary bull's-eye lantern will be found 
the best form of lamp. The barometer, of 
course, should be fixed in a position out of the 
reach of direct sunshine, and not exposed to 
sudden changes of temperature. After the 
barometer, the dry and wet bulb thermometers 
should be read, as in the morning observations, 
and this time the maximum and minimum 
thermometers. The readings of these latter 
should be put down to the day of observation 
(unlike those for the rain gauge, which are set 
down for the previous day). After being read, 
these latter instruments should be set, i.e., the 
detached column of the maximum thermometer 
brought down to the main body of mercury, 
and the index of the minimum (Rutherford's 
form) brought to the top of the column of 
spirit. It is advisable to sometimes examine 
the latter, for occasionally a portion of the spirit 
becomes volatilised and condenses at the upper 



Evening Observations 253 

end of the tube ; it must then be brought back 
to the main column by swinging the instrument 
briskly to and fro, holding it bulb downwards, 
till all the detached portion is united to the 
rest. Sometimes it may be necessary to slightly 
heat the detached portion to make it rejoin the 
rest of the liquid. Casella's minimum thermom- 
eter is also sometimes used, but is a very delicate 
instrument, and the liquid employed being 
mercury, it cannot be used for very low tempera- 
tures, though for such as are met with in these 
islands its readings are probably more accurate 
than those of a spirit thermometer. Alcohol, 
though it has the advantage of remaining liquid 
at the lowest temperatures, is not so regular 
in its expansion and contraction by heat or 
cold as mercury, is more volatile, and not so 
sensitive to sudden changes of temperature. 

After the dry and wet bulb, maximum and 
minimum thermometers are read, and the latter 
set, the black bulb and grass minimum ther- 
mometers should next be examined, their read- 
ings taken, and these instruments then set also. 
Their readings are also to be entered for the 
same day as that of observation, like those of 
the maximum and minimum thermometers. 
Thus each of these instruments is only read 
once a day, in the evening, whilst the barometer 
and its attached thermometer, as well as the 
dry and wet bulb thermometers, are read twice 
daily. Lastly, the card of the sunshine recorder 

Q 



254 Weather Science 

should be removed, and a new card put in for 
the following day. The trace can be measured 
at leisure, by comparing its length with the scale 
of hours marked upon the card, the date during 
which it was exposed being always written upon 
the card immediately after its removal from the 
groove. If this is not done, after a few days it 
will be difficult, if not impossible, to say on which 
day so much, or so little, bright sunshine was 
recorded. As in the morning, the direction and 
force of the wind, the form and amount of 
cloudiness, weather at time of observation, and 
its general character since the morning, should 
be recorded in the observer's rough note-book, 
before he has had time to forget or to confuse 
the weather of one day with that of another. 
An interesting task will be found in the com- 
parison of the actually observed weather during 
any period at the observer's station with the 
general character of that predicted for his district 
by the " Daily Weather Report " issued by the 
Meteorological Office. Speaking only for his 
own observations, the author can testify that 
though a remarkable degree of accordance some- 
times appears between predicted and recorded 
weather, the percentage of predictions of " com- 
plete " or " partial " successes always seemed less 
than that often claimed for these prognostics. 

No doubt this arises from the difficulty of 
assigning and allowing for local variations of a 
more or less temporary character : when there 



Weather Forecasts 255 

may be heavy continuous rain in one station, 
there will only be a few slight showers in a 
neighbouring district; a storm on the exposed 
coast will only be felt as a moderate amount 
of wind a few miles inland, etc. 

Moreover, the whole of the British Islands 
being divided into eleven districts, it frequently 
happens that very different weather prevails in 
different parts of one and the same meteoro- 
logical district. For instance, district No. 5, 
"England, south London and Channel," covers 
a wide area, and it is no uncommon experience 
to find very different conditions prevailing even 
in different parts of London alone. On the 
other hand, especially in settled anticyclonic con- 
ditions, much the same kind of weather prevails 
over a whole continent for days at a time. 

About fifty stations send daily telegraphic 
communications, and from the information thus 
supplied the weather charts are prepared and 
forecasts issued by the London office. It has 
been often pointed out that our own islands 
(and especially the western portions) are not well 
situated for the prediction of probable weather 
changes, and the issue of forecasts. The pre- 
vailing wind, south-west, comes to us from the 
ocean, over which no permanent stations exist, 
and though one sometimes hears of storms " tele- 
graphed " from across the Atlantic, yet it must 
rarely happen that a cyclonic disturbance, even 
starting due eastward from North America, will 



256 Weather Science 

travel without deviation in its course, and reach 
our shores. More often it will change in direc- 
tion whilst passing over so great a distance, or 
may even fill up. On the Continent, in Germany 
and Central Europe generally, the conditions 
for successful forecasting are more favourable, 
since there are land stations both to the east 
and west whence they may receive information 
of coming changes, and the same is the case 
with the eastern states of North America. 

A good many years ago a scheme was 
developed by the late Admiral Fitzroy, then 
Director of the Meteorological Office, for convey- 
ing storm warning to seaport towns. Telegraph 
information of storms in progress, which storms 
might be expected to travel to other districts, 
was received, and the news sent to the threatened 
districts as far as possible. 

The signalling apparatus known as the " cone 
and drum" was set up in various prominent 
positions, and the references such as " N. cone 
flying in districts 2 and 3," etc., still appear in 
the daily "weather forecasts" published by the 
newspapers. 

The signal apparatus consisted of a mast about 
30 to 40 feet high, with cordage, a cone, a drum, 
and signal lanterns. Each cone and drum was 
made of a wooden frame covered with canvas, 
and their size was about 3 feet. When hoisted 
with its point upwards, the cone indicated that a 
gale from the north was to be expected ; if its 
point was turned downwards, a southerly storm 



Cone and Drum Warnings 257 

was coming. This was read north cone flying 
or south cone flying, as the case might be. 
The hoisting of the drum indicated that storms 
might be expected, but direction indefinite. The 
hoisting of both cone and drum indicated a 
violent storm, the position of the cone giving the 
probable direction. "These cautionary signals 
advert to winds during the next two or three 
days." The drum has been discontinued, but 
the cone is still hoisted from time to time, as 
occasion seems to warrant, and though opinion 
seems divided as to the utility of these warnings, 
still "it is better to warn a ship's captain 
erroneously of a storm which does not happen, 
than not to give him a warning of a storm 
which does come." However, Mr Chambers 
states that in 1894 as many as 92 per cent, of 
the warnings were followed by gales or strong 
winds, as against 87 per cent, in 1890 and 79 
per cent, in 1885. Thus it would appear that 
accuracy in this respect is increasing. 

The eleven districts into which the British 
Islands are now divided for " forecasting " 
purposes are as follows : 

District No. District 

0. North Scotland (north of Caledonian Canal). 

1 . East Scotland (south of Caledonian Canal, Aberdeen, 

Perth, Lothians, etc.). 

2. North-East England (Northumberland, Durham, 

Yorkshire, Lincolnshire). 

3. East England (East Anglia, Essex). 

4. Midland Counties (Bucks, Oxford, Warwick, 

Stafford, Derby, etc.). 



258 Weather Science 

District No. District. 

5. England, South London and Channel (Kent, 

Surrey, Sussex, Hampshire, etc.). 

6. West Scotland (south of Caledonian Canal, Argyll, 

Ayr, Galloway, etc.). 

7. North- West England (Cumberland, Westmorland, 

Lancashire, North Wales). 

8. South- West England (Devon, Cornwall, Somerset, 

South and Central Wales). 

9. North Ireland (Ulster, North Connaught, and 

North Leinster). 

10. South Ireland (rest of Ireland, south of latitude 
N.). 



But two or more of these are often combined, 
and a smaller number of divisions is probably 
in general sufficient, so far as the accuracy of 
the forecasts at present goes, since only very 
general information can be given. 

The Meteorological Office, London, at present 
issues daily and weekly weather reports. On 
the daily report (one sheet of four pages, price 
one penny, issued for 8 A.M. of each day) is given 
(page 1) the barometric pressure, temperature, 
direction and force of wind, and cloudiness, 
at over fifty stations in Scandinavia, France, 
Germany, and the British Isles (and occasion- 
ally one or two in Spain), for the previous 
evening and the morning of the day of issue, 
with the changes in pressure and temperature, 
the maximum and minimum temperatures during 
the past twenty-four hours, hours of sunshine and 
amount of rainfall, etc. On the second page 
are given charts giving the isobars at intervals 
of an inch for the morning of issue, the 



Weekly Weather Report 259 

temperature and other weather conditions, also 
a map showing the average rainfall and mean 
temperature during the same period as deduced 
from long continued observations. General 
remarks on the " situation " and possible changes 
are discussed on page 3, and the " forecasts " for 
the twenty-four hours, ending at noon the day 
following that of issue, are given. On the last 
page of the sheet additional observations, late 
reports for the previous day, etc., are entered. 1 

The weekly weather report, as its name indi- 
cates, gives on six pages or so a summary of 
the weather conditions over these islands and 
the greater part of Europe for the past week. 

A summary of temperature, rainfall, and 
duration of bright sunshine in each of the eleven 
districts and the Channel Islands is first given, 
also comparisons with and differences from the 
mean values of these quantities and "general 
remarks." Next follow the data from which 
the summary has been calculated, the weekly 
reports from about eighty first and second order 
stations in the eleven districts. For each day 
during the week is given a chart of temperature 
and weather at 8 A.M., and barometer and wind 
at 8 A.M. and 6 P.M. respectively, with remarks 
on changes at foot of each chart. At the end, 
explanation of symbols, isobars, arrows to show 

1 From 1st January, 1911, some alteration has been made in the 
<( Daily Weather Report." Pages 2 and 3 are recast, the maps 
enlarged, and additional information given. A new section, ' * London 
Observations," appears on page 4, in addition to the additional 
observations, radio telegraphic information from the Atlantic, etc. 



260 Weather Science 

the force of wind (thus O calm ^ light to 

moderate breeze, ^ ^ fresh to strong breeze, 
^ > gale, etc.), and the weather abbrevia- 
tions used, e.g. 9 6 = blue sky, c = detached clouds, 
o = overcast, m = misty, f foggy, q = squally, 
r = rain, h = hail, s = snow, / = lightning, t 
thunder. Isothermal lines show the distribution 
of temperature and diagonal lines = rough sea, etc. , 
with shading proportional to the disturbance. 

Note. Although the records of temperature, pressure, etc., are 
commonly given in Fahrenheit degrees and ' { English " measure, 
many recent meteorological investigations are quoted in terms of 
the metric system and centigrade scale of temperatures, but the 
conversion from one to another is merely a matter of arithmetic. 
Though there seems a prejudice against such numbers as 15 for 
"temperate/' 35 for "blood heat/' etc., yet the advantage of a 
common scale, universally accepted and requiring no arithmetical 
manipulation to be at once available, is such that trifling difficulties 
of that kind will be soon got over. Greenwich time has been almost 
universally adopted, even recently by the French, so that it would 
be a graceful act on the part of British meteorologists to use the 
measures sometimes called " French," but which are not merely so, 
whose symmetry and uniformity contrast so strongly with the 
complexity of our "British" systems, the acquisition of a knowledge 
of which takes up so much of the schoolboy's time, most of which 
knowledge is soon forgotten afterwards. 

Note. By the kind permission of my friend, Mr Inwards, I give 
an illustration of the Temple of the Winds, Athens, which may 
perhaps be called the oldest meteorological observatory in the 
world. This building is a small marble octagonal tower, and was 
constructed about 100 B.C. At the centre of the roof was placed 
a wind-vane, the figure of a triton, whose ( ' sceptre " always pointed 
to the ( ( wind octant." The eight sides of this temple were built 
so as each to face one of these directions, and on each side was 
sculptured a human figure in relief, representing the character of 
a particular wind. The north wind is represented by a figure of a 
warmly-clad man clothed in furs, and blowing fiercely on a trumpet ; 
the east wind is expressed by the figure of a young man with flowing 
hair ; the west wind by the figure of a lightly-clad and beautiful 
youth, with his lap full of flowers. Each figure represented, ' ' as 
far as a figure can, the character and qualities of the particular 
wind which it faces," and thus we have evidence that the character 
of the winds has not materially changed during the last two 
thousand years in Greece, 




TEMPLE OF THE WINDS, ATHENS. 
(By permission of R. Inwards, Esq., F.R.A.S.) 



To face p. 260. 



Weather Signs and Portents 261 



CHAPTER XIV 

WEATHER SIGNS AND PORTENTS OF COMING CHANGES SUN MOON 

CLOUDS CIRRUS CIRRO-STRATUS CIRRO - CUMULUS STRATUS 

CUMULUS CUMULO-STRATUS COLOURS OF THE SKY ANIMALS 

BIRDS. 

THE accumulated experience of centuries of 
observation has led to the discovery of certain 
rules and weather signs ; a brief summary of a 
few of these may not be without interest. First 
and foremost we may put such as are associ- 
ated with the appearance of the sun and moon ; 
then come the clouds, whose form, density, and 
motion are so continually varying ; and lastly, 
signs derived from the movements of animals, 
opening and closing of plants, etc. Of these we 
may remark that the influence and appearance 
of the sun has a very real bearing universally 
recognised, but the supposed changes of weather, 
due to changes of the moon, though widely 
believed in, " without rhyme or reason," rest 
on no basis of fact at all. In one sense the 
moon is never changing, in another it is always 
changing. The transition from " New Moon " 
to "First Quarter," etc., the varying appear- 
ance called the " change " of the moon, is a 



262 Weather Science 

gradual phenomenon, and merely represents a 
greater or less amount of its illuminated surface 
turned towards us. The moon, being an opaque 
globe, shines only by reflected light from the sun, 
and the latter only lights up one hemisphere 
at a time, more or less of which is turned 
towards the earth at different times. Since the 
total amount of light given by the full moon 
to the earth is only about su^yooz^h tnat f 
sunlight, and the heat is even more difficult to 
detect, any action must be of excessively small 
amount. 

Moreover, the evidence of statistics is con- 
clusive as to the absence of any connection 
between the moon's phases and the state of 
the weather, though some are of opinion that 
the full moon has a slight clearing effect on a 
cloudy sky. This is expressed by the saying : 
"The full moon eats clouds." 

A red sky in the evening, followed by a grey 
sky in the morning, is generally regarded as a 
sign of fine weather, and conversely a grey sky 
at night, followed by red, lowering weather 
next morning, is considered a sign of coming 
rain. This opinion is expressed in the popular 
lines : 

" Evening red and morning grey, 
Help the traveller on his way. 
Evening grey and morning red, 
Pour down rain upon his head." 

The appearance of sun rays or streaks between 



Halos and Coronae 263 

the clouds indicates rain to follow soon, and is 
popularly called "the sun drawing water." 

The appearance of halos round sun or 
moon furnishes a copious literature, and the 
popular idea is referred to by poets and others. 
Longfellow, in his Wreck of the Hesperus, 
makes a sailor say to the skipper: 

" I pray thee put into yonder port 
For I fear a hurricane. 
Last night the moon had a golden ring, 
And to-night no moon we see ! " 

The greater or less distance of this halo or 
corona from the moon is supposed to indicate 
the less or greater time within which the rain 
will fall: 

" Circle near, water far, 
Circle far, water near." 

The size of the drops regulates the diameter. 
The colour of the sky towards sunset is a 
prognostic of wind. "A bright yellow sky at 
sunset presages wind " (Fitzroy). A pale yellow 
sky is a sign of wet : 

" If the sun goes pale to bed, 
It will rain to-morrow, it is said." 

This "watery" sun is a very common indica- 
tion of the front part of a cyclone. " After the 
front part, where the sky gives a watery look 
to the sun, has passed over the observer, the 
rainy portion will also have to come over 
him." 



264 Weather Science 

Amongst imaginary signs which appear to 
have gained considerable credence, one knows 
not why or how, may be mentioned the " moon 
on her back," with horns pointing upwards, so 
often regarded as a presage of wet weather. 
The position of the moon's "horns" entirely 
depends on her angular distance from the sun, 
the line joining the points, or cusps, as they are 
called, being always perpendicular to the line 
joining sun and moon, and has consequently 
nothing whatsoever to do with the weather, 
being always definite and predictable before- 
hand. The moon's " changes " have an equally 
fictitious connection with weather changes. As 
we have said, in one sense the moon is always 
changing, the amount of light reflected by her 
towards the earth continuously increasing from 
new moon to full moon, and continuously 
decreasing from full moon to new moon again, 
but the whole moon is always there, so that 
apart from the slow change of distance we may 
say that the moon never changes ! Since the 
total amount of light given us by the full 
moon does not exceed ^nriWukh tnat f sun " 
light, we see how excessively small must be 
the effect of changes in so insignificant a 
quantity. 

The moon, by its gravitative action, must 
produce a minute atmospheric tide analogous 
in kind to that produced by her in the waters 
of the ocean, but its amount is so small that 
even in tropical regions this "tide" is scarcely 



Cloud Lore 265 

detectable, whilst in other parts of the world 
the smallest disturbance due to other causes 
completely masks it. Immense masses of 
statistics have been produced to show the want 
of connection between moon " changes " and 
weather changes, but the legend still lingers on, 
perhaps one justification of Carlyle's well-known 
classification of the population of these islands. 

There is a very extensive literature of cloud 
lore. From the earliest ages to the present 
time the weather signs and warnings prevalent 
in all countries have been of great value in 
general, though occasions arise when the event 
belies the prognostic. Many references to cloud 
appearances occur in the Bible, both in the 
Old and New Testament : " A little cloud out 
of the sea like a man's hand," the forerunner of 
a storm. "When ye see a cloud rise out of 
the west, straightway ye say : There cometh 
a shower, and so it is " (St Luke xii. 54). 
Admiral Fitzroy's remarks (chap, vii.), already 
referred to, may be supplemented by similar 
quotations from the "Shepherd of Banbury," 
and other weather prophets. " The increase of 
cloud indicates rain," whilst conversely " cloudy 
mornings often change to clear evenings." The 
movements of upper clouds crossing the sun 
and stars in a direction different from that 
of the lower clouds and wind, foretell a change 
of wind, whilst in hot weather the appearance 
of two strata of clouds moving in different 
directions is a sign of coming thunder. The 



266 Weather Science 

change of cirrus clouds in the windward to 
cirro-stratus is a sign of rain. The appearance 
of these loftiest clouds, after a continued spell 
of fine weather, is a very certain prognostic of 
coming change, though small groups scattered 
over the sky are often found with fairly settled 
weather conditions. " Long parallel bands in 
the direction of the wind indicate steady high 
winds to come." Howard remarks of cirrus 
clouds : " These clouds announce the east wind. 
If their under surface is level and their streaks 
pointing upwards, they indicate rain, if down- 
wards, wind and dry weather." After a clear 
frost, long streaks of cirrus, with ends bending 
towards each other, and pointing to the north- 
east, are signs of a thaw and south-west wind. 
Cirrus clouds in detached tufts, " mare's tails," 
as they are sometimes called, are regarded as 
a sign of coming wind, which blows from the 
quarter towards which the " tails " have pointed. 
The cirro - stratus form of cloud, fish - shaped, 
or "very like a whale," if pointing east and 
west indicates rain; if north and south, more 
fine weather (Inwards). 

" If clouds look as if scratched by a hen, 
Get ready to reef your topsails then/' 

This is in accordance with the opinion tha 
this form of cloud precedes winds and rains, anc 
the approach of bad weather is inferred frorr 
its greater or less abundance. The wavy-formec 
cirro-stratus is a forerunner of heat and thunder 



Mackerel Sky 267 

The phenomena of coronas, parhelia and para- 
selenae (" mock suns " and " mock moons "), occur 
with this form of cloud owing to its great extent 
and evenness, with little depth (thickness), so 
; that these phenomena come to be themselves 
regarded as indications of the type of weather 
experienced when cirro - stratus clouds are 
present. 

The cirro-cumulus cloud is well known from 
its producing the appearance known as mackerel 
I sky, whose prognostics are summarised in the 
lines : 

" Mackerel sky, mackerel sky, 
Not long wet, nor yet long dry." 

In connection with the cirrus "mare's tails" 
we have the nautical saying: 

" Mackerel sky and mare's tails 
Make lofty ships carry low sails," 

thus being an indication of coming wind. Cirro- 
cumulus clouds at a considerable height often 
indicate thunderstorms in a few hours, or 
perhaps longer. " Before thunder cirro-cumulus 
clouds often appear in very dense and compact 
masses in close contact" (Inwards). If, on the 
other hand, the clouds are soft and delicate in 
outline, fine weather may result, lasting for 
some few days. 

The stratus or " night " cloud, as it is some- 
times called, from its usual formation towards 
evening and dissipation by sunrise, is usually 



268 Weather Science 

regarded as a harbinger of fine weather " there 
are few finer days in the year than when the 
morning breaks out through a disappearing 
stratus cloud." In calm weather some fog at a 
lower level usually persists till dissipated by the 
increasing heat of the sun, " ground fog," indeed, 
being a name often applied by Howard to low 
stratus clouds. The cumulus or "day" cloud, 
when high up, is said to show that south and 
south-west winds are near at hand. Before rain 
these clouds increase rapidly in size, sink, and 
become more irregular in shape ; if, on the other 
hand, they become "smaller at sunset than 
they were at noon," fine weather may be ex- 
pected. If they are formed to " leeward " during 
a strong wind, they indicate the approach of a 
calm with rain, if they become heaped up during 
a strong wind at sunset thunder may be expected 
in the night (Inwards). The edges being very 
white, and the clouds fleecy and dense, or thick 
and close towards the middle, whilst the sky 
around is blue, they are of a frosty coldne* 
and will speedily fall in hail, snow, or rail 
Captain Wilson Barker, in his " Clouds an< 
Weather," distinguishes five varieties of cumulus : 
(1) the fine weather cumulus; (2) the roll 
cumulus, seen towards the close of stormy 
weather; (3) the squall cumulus, accompanied 
by rain, snow, thunder and lightning, etc., 
according to season ; (4) pillar cumulus, a rare 
form seen only in equatorial regions ("the 



Colours of the Sky, etc, 269 

doldrums ") ; (5) shower cumulus, from which 
fine showers fall, but which is accompanied by 
little or no wind. 

The cumulo-stratus " the cumulus, as it were, 
changing into the nimbus" (Scott) is thus a 
forerunner of rain or snow, according to the 
season. Large masses collecting in the north- 
east and south-west, with the wind east, then cold 
rain, or snow may be expected, the wind ulti- 
mately backing towards the north (Chambers). 

The varying colours of the sky and of the 
clouds have also bearings on the probable future 
weather conditions. Admiral Fitzroy remarks : 
" A dark, gloomy, blue sky is windy, but a light, 
bright, blue sky indicates fine weather ; when the 
sky is of a sickly-looking greenish hue, wind or 
rain may be expected." Unusual visibility of 
distant objects, the outlines of hills seen sharply 
defined, distant sounds distinctly heard, are 
usually regarded as signs of coming rain. Doors 
and windows creak, blind cords snap, and persons 
afflicted with rheumatics or with old wounds and 
sores complain of more than ordinary pains. 
Animals in general, whose very existence often 
depends on slight changes in the weather, are 
especially sensitive to and cognisant of approach- 
ing rain or storms long before we know of 
their coming by other indications. Some of the 
"signs" of rain given are no doubt the result 
of careful observation of the habits of the lower 
animals, but it is difficult to say what value 



270 Weather Science 

is to be attached to such tokens as the 
following : 

" When cats sneeze, it is a sign of rain ! " 
" When dogs eat grass, it will be rainy ! " 

The latter habit is by some authorities stated to 
be a medicinal precaution on the part of our 
canine friends. 

The flight of birds, far and wide in fine weather, \ 
short and staying near their nests in more un- 
certain conditions, has been alluded to, and in 
most cases may be considered a reliable indica- 
tion of the goodness or badness of weather for 
the next few days. 

A disturbed condition of the animal world 
generally, the huddling of sheep together, loud 
croaking of frogs, chattering of magpies, crows, 
sparrows, etc., are all well-known signs of coming 
rain. A cloudy, gloomy day is preferred by the 
angler, "fishes rising more than usual at the 
approach of a storm," though we are also told 
that "they are said (in some parts) not to bite 
so well before rain." Bees are also notoriously 
fine weather animals, not venturing out when 
it is likely to be rainy, in accordance with the 
lines : 

" If bees stay at home, 
Rain will soon come. 
If they fly away, 
Fine will be the day." 

Even more sensitive than animals, the plant 



The Ploughman's Weather Glass 271 

world is full of weather prophets. We have the 
well-known pimpernel, or Ploughman's weather- 
glass, the sea- weed, whose hygroscopic properties 
j are often made use of, sensitive plants whose 
leaves contract at the approach of rain, and count- 
less other objects. 

The opening and closing of the petals of the 
pimpernel "is better understood amongst the 
Bedfordshire labourers than the indications of 
any instrument." In fine weather it opens in 
the morning (usually from 7 to 8 A.M.) and closes 
in the afternoon (2 to 3 P.M. ) ; if it closes earlier 
than usual, or fails to open in the morning, this 
may be regarded as a sure sign of approaching 
rain. " Closed is the pink-eyed pimpernel." 

The leaves of many trees curl more or less 
when the air is damp. " When the down of the 
dandelion contracts, it is a sign of rain ; " whilst 
we are told conversely, " When fine weather is 
to follow, chickweed expands its leaves boldly 
and fully." 



CHAPTER XV 

CYCLES OF WEATHER BRUCKNER'S INVESTIGATION RAINFALL WET 

AND DRY PERIODS OSCILLATIONS OF MEAN PRESSURE AND 

TEMPERATURE SUN-SPOTS AND THE WEATHER SUN-SPOT 

" PERIOD " I ITS IRREGULAR LENGTH THE MOON. 

THE recurrence of cycles of hot and cold years, 
years of unusual dryness or of excessive precipita- 
tion, has often been asserted, but it is only within 
the last twenty years or so that any definite 
scientific information has been obtained from the 
vast masses of material collected all over the 
globe. 

Nearly three hundred years ago Lord Bacon 
expressed the opinion that every forty years the 
same kind of weather recurred, and gave forth 
aphorisms such as the following, based on a sort 
of instinctive appreciation of the law of averages : 
"A serene autumn denotes a windy winter; 
a windy winter, a rainy spring ; a rainy spring, 
a serene summer; a serene summer, a windy 
autumn," so that the air on the balance is seldom 
debtor to itself (Inwards, " Weather Wisdom "); 
but these and similar "saws " are more often 
honoured in the breach than in the observance. 
The recurrence of former periods of unusual cold 

272 



Cycles of Weather 273 

(glacial epochs), and of intermediate times of 
j more moderate temperature, is well established 
! by geological observations, and theories as to 
their cause, intermission, and recurrence have 
I been propounded by astronomers and geologists 
j from time to time, none, however, receiving 
i general support. Of more recent and immediate 
i importance, however, is the question of annual 
! fluctuations in temperature and rainfall now 
i going on. In 1891 Professor Bruckner pub- 
] lished the results of his investigations from the 
I records of over three hundred stations dis- 
tributed throughout the five continents, for the 
period 1830-1885 ; in some cases for a much 
longer period (e.g., the observations at Paris 
go back to the end of the seventeenth century). 
Dividing his results into five year periods, or 
lustra, he found that for Europe the years 
1831-1840 were on the whole years of deficient 
rainfall ; for the period 1841-1855 the amount 
of rain was in excess of the average ; 1856-1870 
j was again a period of deficiency, and 1871-1885 
i one of excess. The other continents show 
similar results, but for Asia the deviation of 
individual lustra from the mean rainfall is 
greater than for other parts of the globe. In 
the driest period the rainfall is only about 
three-quarters its value in the wettest. This 
oscillation is general throughout the globe at 
the same time, so that a deficient supply in 
one continent is not counterbalanced by an 



274 Weather Science 

excess in another. The variation in the level 
of inland seas, lakes, rivers, etc., also affords a 
measure of long period variations in rainfall, 
and Dr Bruckner gave the average duration 
of such oscillation for the Caspian Sea as about 
thirty-five years. From records of the Alpine 
glaciers and lakes in Europe, ten lakes in 
North America, and twelve lakes in Asia, he 
found that in general the periods of high and 
low water occur simultaneously all over the 
world, and the average value of the time from 
one maximum or minimum to the following 
one to be about 35*6 years. The times of 
" minima," for a number of rivers and lakes 
examined by him, are as nearly as possible 
the times of dry and warm climate, whilst 
the maxima correspond with wet and cold 
weather. 

Coming now to variations of temperature, 
Bruckner computed " lustra means " for twenty- 
two regions, using also some data given by 
Koppen for twenty-nine districts, mainly in the 
Northern Hemisphere. He thus obtained the 
following periods of relative heat and cold for 
a century (1791-1890): 



Warm period, 1791-1805. 
Cold 1806-1820. 
Warm 1821-1835. 



Cold period, 1836-1850. 
Warm 1851-1870. 
Cold 1871-1885. 



The reality of these periods is accepted by 
many meteorologists, and they are accordingly 



Bruckner's Periods 275 

known by the name of Bruckner's hot or cold 
periods respectively. It must be confessed, 
however, that the actual excess or defect of 
temperature during any of these periods, as 
referred to the general mean for the whole time 
that observations have been made, is but small, 
and some portion of the material used by 
Bruckner and Koppen gave indefinite results. 

Bruckner, using the materials given by Hann 
in his book (" Luftdruck in Europa "), has also 
gone into the question of periodic variations 
in barometric pressure. His results are sum- 
marised by Dr Waldo (" Modern Meteorology") 
as follows : 

For the dry period as compared with the 
rainy period there exists : 

(1) A deepening of the constant cyclone 

which the annual averages show for 
the North Atlantic. 

(2) An increase of the high pressure which 

extends from the Azores to the interior 
of Russia (anti- cyclone). 

(3) A deepening of the low pressure in the 

northern part of the Indian Ocean and 
China Sea (cyclone). 

(4) A decrease of the high pressure over 

Siberia (anti-cyclone). 

(5) A general increase in the amplitude of the 

yearly oscillation, which causes in the 
dry period in winter high pressure in 
Europe and Siberia, low pressure over 



276 Weather Science 

the North Atlantic, and in summer a 
relatively low pressure in Central and 
Western Europe and on the North 
Atlantic. 

Thus each rainy period is accompanied by a 
diminution of the differences of pressure exist- 
ing over different regions, and conversely a dry 
period accentuates these differences, both for 
the annual averages at different places and also 
for seasonal average at the same spot. 

The oscillations of " ice periods," i.e., the 
variations in the length of time when the rivers 
of Northern Europe and Asia are ice-bound, 
and the dates of their becoming free from ice 
in the spring, as well as the extended series of 
records giving the times of the grape harvest 
in Germany, Switzerland, and France for several 
centuries, have also been examined by Dr 
Bruckner. By means of the latter he has found 
it possible thus to estimate the periods of unusual 
warmth or dryness and those of cold and wet 
for Central Europe during nearly five hundred 
years. 

By making use of records of the occurrence 
of severe winters from a yet earlier date, it has 
been found, with a fair degree of probability, 
that an average period of about thirty-five years 
intervenes between one succession of excess or 
deficiency of warmth and the following one, and 
this period we have seen also appears in dealing 
with variations of rainfall, water level, and baro- 



Schwabe and Sun-spots 277 

metric pressure. Thus, after all, Bacon's guess 
as to the recurrence of weather after forty years 
seems not so far out. 

Of another kind is the question of the con- 
nection between sun-spots and the weather. 
More than fifty years ago Hofrath Schwabe, 
! of Dessau, who had long and carefully observed 
the surface of the sun through his telescope, 
showed that the dark markings on the solar 
surface are periodic in character, at times being 
large and numerous, at other times almost or 
quite absent from the disc, and that the average 
interval from one maximum to the next is 
about eleven years. This period is, however, 
only a rough approximation, being sometimes 
as long as sixteen years, at other times as short 
as eight years. There seems also some evidence 
of the existence of secondary maxima and 
minima superposed in the main course. The 
nature of these spots is even yet quite uncertain, 
though there is no doubt that many, if not most, 
of them are depressions of some kind or other. 
The presence of these dark markings of itself 
diminishes the total amount of sunlight received, 
but on the other hand their existence indicates 
a generally disturbed condition of the solar 
photosphere (or light-giving envelope), so that 
it is quite uncertain whether we receive more or 
less heat from the sun at the time of a sun-spot 
maximum. The difference in any case must be 
a very minute fraction of the whole amount. 



278 Weather Science 






Of a connection between the greater or less 
frequency of spots and their sudden appearance 
and disappearance with terrestrial magnetism 
there can be no doubt. 

The irregular variations of the needle, known 
as magnetic storms, are almost invariably syn- 
chronous with outbursts on the solar surface. 
Brilliant exhibitions of the Aurora (Borealis or 
Australis) are also seen over wide areas. There 
is, in addition to these occasional and spasmodic 
apparitions, a more intimate connection between 
the general variations of magnetic intensity and 
the great or less spottedness of the sun. Lamont, 
in 1850, pointed out that the average daily move- 
ments of the magnetic needle have a period of 
about ten and a half years, and that the greatest 
of these diurnal oscillations from mean position 
happen at or near the times of sun-spot maxima. 
In addition to its general pointing north and 
south, the compass needle undergoes minute daily 
changes. From about 7 A.M. it travels westwards 
through a small angle till about 1 P.M., during 
the afternoon it returns eastward till about 
10 P.M., and then remains quiet till the morning 
again, but in summer-time the needle begins 
to move again slightly westward to about mid- 
night, and returns again eastwards before 7 A.M. 
(Thompson). The extent of these oscillations 
does not usually exceed about 10' of arc, being 
slightly greater in summer than in winter, and the 
average extent of this oscillation increases and 



Sun-spots Terrestrial Temperatures 279 

decreases with fair regularity during a period 
of about eleven years or so. 

Attempts have been made by various investi- 
gators to ascertain any connection between sun- 
spots and any other terrestrial phenomena, but 
so far the verdict must be " not proven " (see 
also chap. xi.). Jelinek, in 1870, from all 
records of temperature available in Germany 
up to that time, " found the influence of 
sun-spots inappreciable," whilst Stone at the 
Cape and Dr Gould of Cordoba, in South 
America, considered that observations taken 
at their stations showed a distinct though 
slight diminution of temperature at the time of 
sun-spot maximum. On the other hand, Mr 
F. Chambers of Bombay, in a paper contributed 
to Nature during 1878, drew the conclusion 
from barometer observations between 1848-1876 
that the sun is hottest when most spotted. The 
late Dr Meldrum of Mauritius, from a com- 
parison between the number of cyclonic storms 
observed in the Indian Ocean and the " spotted- 
ness " of the sun, considered that the former 
were more frequent at the time of sun-spot 
maximum than at those of minima, but the 
evidence on which this conclusion rests is of a 
somewhat unsatisfactory character. Dr Meldrum, 
and later, Sir Norman Lockyer, have made com- 
parisons of the rainfall at stations near the Indian 
Ocean, Cape of Good Hope, and in India, which 
they considered to be, on the whole, confirma- 



280 Weather Science 

tory of this connection. On the other hand, the 
results from American stations seem to indicate 
that on the whole somewhat less rain than usual 
falls during a sun-spot maximum. The matter 
was very thoroughly discussed by the late 
Mr Symons, the greatest authority on rainfall 
of his day, and he concluded that there was 
no certain evidence of any connection. Mr 
Maunder, of Greenwich Observatory, from an 
examination of Sir John Eliot's and Dr 
Meldrum's catalogues of cyclones in the Bay 
of Bengal and the Southern Indian Ocean, has 
found several striking instances of the recurrence 
of a " cyclone " (cyclonic storm) at the interval of 
the solar synodic rotation (about twenty- seven 
and a quarter days). Of one hundred and nine 
cyclones recorded, he finds one sequence of five 
cyclones following each other at the above- 
mentioned interval, and several other sequences 
of four, though in some cases an interval of two 
or more rotations occurred between one storm 
and the following. He points out, however, 
that though this evidence be sufficient to render 
probable a connection between this rotation 
period and the frequency of cyclones, it does 
not necessarily follow that these storms are 
directly connected with sun-spots. The latter 
phenomena are those most easily observed, but 
it does not therefore follow that " sun-spots are 
in themselves the most significant of solar 
phenomena." In any case, the relationship 



Sun-spots and Commercial Crises 281 

pointed out indicates the connection of an 
"interval" and not a period. 

The late Professor Jevons and others have 
tried to deduce some relation between sun-spots 
and periods of commercial crises, arguing that 
if the former have any effect on temperature, 
frequency of storms, and rainfall, they must thus 
indirectly affect agriculture and the state of the 
crops, etc., but whilst there is still so much 
uncertainty as to the nature of the relationship 
between the more or less spotted condition of 
the sun's surface and terrestrial meteorology, we 
must regard all such attempts as premature. 

In conclusion we may quote the words of 
Abercromby ("Weather," p. 325) : 

"It is no doubt a very tempting ideal to 
look at the sun as the prime mover of the 
atmosphere, and to endeavour to follow varia- 
tions in the heat or energy of his action into 
their final products as wind or rain. But when 
we consider what the real nature of weather is, 
as revealed to us by means of synoptic charts, 
we see at once that, though undoubtedly an 
alteration in the sun's power would sooner or 
later be reflected in his results, any attempt to 
deduce one from the other directly must lead 
to disastrous failure." 

The subject of lunar influence on the weather, 
so favourite an idea with many, needs little 
notice here ; it is rightly called by the late 
Professor Young "a relic of ancient superstition." 



282 Weather Science 

It is next to impossible to say what difference 
of temperature is produced by the moon's heat- 
ing action. The moon in one sense is always 
changing, in another always the same. In fact, 
in the words of the " poet," we may say : 

" The moon and the weather 
Oft change together ; 
But change of the moon 
Does not change the weather." 

M. Flammarion estimates that the heat reach- 
ing us from the moon only affects the tempera- 
ture by twelve millionths of a degree, and the 
atmospheric tide in the barometric pressure by 
a few hundredths of an inch. Statistics and 
weather tables examined from time to time by 
various authorities all give negative results. It 
is somewhat curious, to say the least, that those 
who assert this action of the moon, microscopic 
as it must be, frequently ignore the real and 
undoubted influence of the sun, the prime mover 
in all terrestrial phenomena. If, however, we 
take the so-called " changes " of the moon, these 
occur once every seven or eight days, so that 
all changes of the weather must occur within 
three or four days one side or the other of these 
events, and it is not difficult to find a sufficient 
number occurring within a less time. Thus, 
by ignoring all the rest, it is easy to establish 
such a connection as is desired, and indeed 
almost any desired relation can be similarly 
arrived at. 



Planetary Influences 283 

Mr Inwards, in his " Weather Fallacies," gives 
other lunar superstitions, such as " Two full 
moons in one month cause a flood " ; " It is a 
bad sign if the moon ' changes ' on Saturday 
or Sunday," etc. Even the " halo " round the 
moon does not always precede rain, but this 
appearance, like that of the " old moon in the 
new moon's arms," stands on a higher level 
than the lunar superstitions, whilst no less 
an authority than Sir John Herschel believed 
that the full moon possessed a slight power of 
clearing away clouds. This has already been 
referred to in an earlier part of the present 
work. 

Since the days when astrology was seriously 
studied, however, few have asserted any direct 
influence of the planets upon weather conditions, 
though the curious medieval idea that dew was 
a product of the stars may be even yet not 
extinct. One remembers, however, that some 
years ago a would-be " weather prophet " claimed 
to predict the future state of terrestrial weather 
from the position and motions of the planets 
Jupiter and Venus ! When his predictions were 
not verified, he asserted that it was owing to 
the inaccuracy of Le Vernier's tables of those 
planets, upon which he had relied! Whether 
more exact tables enabled him to arrive at 
better results, I did not learn. But the 
words of Sir William Herschel, one hundred 
years ago, still hold good : " Prognostications 



284 Weather Science 

of the weather are far above the knowledge of 
astronomers " ; all that can yet be done is to 
offer probable "forecasts" for two or three 
days, sometimes less and sometimes more, 
ahead of date. But the mind of man seems 
so constituted that any speculation, however 
absurd, any guess, however random, will find 
supporters, and "fools rush in where angels 
fear to tread." 



Meteorology and Astronomy 285 



CHAPTER XVI 

THE SEASONS HEATING ACTION OP THE SUN ZONES CLIMATE AFFECTED 
BY PROXIMITY TO SEA ISOTHERMAL LINES I THEIR COURSE ACROSS 

THE GLOBE EXTREMES OF TEMPERATURE ISABNORMALS SEA 

TEMPERATURES INFLUENCE OF GULF STREAM, ETC. MINIMUM 
TEMPERATURE OF SEA. 

THERE is probably no branch of meteorology 
more promising at the outset, yet more complex 
and disappointing in the sequel, than the study 
of the laws governing the motion of the atmos- 
phere, and the attempt to infer from its present 
and past condition the probable future state 
of affairs at any given time. In this respect 
the science offers a remarkable contrast to the 
kindred one of astronomy. In the latter the 
accumulated observations of the past having led 
to the discovery of the simple law of gravita- 
tion, it has become possible by means of this 
single principle (combined with the laws of 
motion discovered experimentally by Galileo, 
and first stated in a definite form by Newton, 
to whom also the exact quantitative law of 
gravitation is due), to state with the utmost 
precision the past and future position of the 
sun and planets of the solar system through 



286 



Weather Science 



countless ages. On the other hand, it is a 
matter of common remark how frequently even 




Mean Annual Isotherms for the British Isles. 
(After Buchan and Mill). 

prognostics of weather made one day ahead fail 
signally, and it is probably impossible, even in 



Primary Agencies 287 

the most settled conditions, to make forecasts 
as to their continuance, or otherwise, for more 
than two or three days, at least in our climate. 
Yet the laws which govern the motion of our 
atmospheric ocean, at the bottom of which we 
live and move and have our being, are known, 
and as definite as the law of gravity. The sun's 
heat and the rotation of the earth on its axis 
the former the cause of the great atmospheric 
currents polewards and equatorwards respec- 
tively, the latter the agency producing the main 
deviation in their direction are as regular and 
invariable as any known phenomena, though 
there is no doubt that in the course of ages 
both of these agencies are subject to change. 
Yet so far as the whole recorded history of 
mankind goes, there is no reason to suspect that 
the total amount of heat received annually by 
the earth from the sun has undergone any 
material change, nor has the period of rotation 
(length of day) changed so much as a second 
of time. 

Theories as to the cause of former Ice Ages 
and warmer (interglacial) periods lie outside the 
range of meteorology, as is also the case with 
theories of tidal evolution, etc., based on sup- 
posed variations in the earth's rate of rotation 
in past and future ages. The only extra- 
terrestrial body of whose influence on the atmos- 
phere we have undoubted proofs, is the sun, and 
this influence has not appreciably varied for 



288 Weather Science 

thousands of years ; for though in recent days, 
as we have seen, attempts have been made 
to connect the greater or less spottedness of 
the solar surface with variations in terrestrial 
atmospheric conditions, little or no positive 
evidence has yet been obtained of any such 
connection. 

The sun's thermal influence, however, on the 
whole earth, atmosphere and all, is so prepondera- 
ting and vital, that mankind have never been 
ignorant of the fact, and solar worship is the 
most rational form of superstition, still forming 
the basis of many creeds in civilised lands. In 
a material sense we are truly, as the ancient 
Peruvians called themselves, "the children of 
the sun." 

" The sun's rays," says Herschel (" Outlines of 
Astronomy," p. 399), "are the ultimate source of 
almost every motion that takes place on the 
surface of the earth. By its heat are pro- 
duced all winds, and those disturbances in 
the electric equilibrium of the atmosphere which 
give rise to the phenomena of lightning, and 
probably also to those of terrestrial magnetism 
and the aurora. ... By them the waters of the 
sea are made to circulate in vapour through the 
air and irrigate the land, producing springs and 
rivers. ..." 



With the exception of tidal action (i 
due to the sun, but in a greater degree 



Variation causing Seasons 289 

the moon) and possible magnetic and electrical 
phenomena, the sun's heat is the exciting cause of 
almost all terrestrial motions. The moon, by its 
differential gravitative action upon the atmos- 
phere and the solid earth, produces a minute 
atmospheric tide, causing a rise and fall of 
the mercurial barometer amounting to a few 
thousandths of an inch, and this, with the excep- 
tion of a possible slight disturbance of terrestrial 
magnetism connected with its approach or recess 
at perigee and apogee respectively, is the only 
ascertained influence of that body on terrestrial 
weather conditions. 

The variation in the amount of heat received 
from the sun at different times of the year 
produces the well - known phenomena of the 
seasons. In our latitude this variation (apart 
from the greater or less cloudiness of the sky), 
primarily arises from two "main causes: (1) the 
greater or less altitude at which the sun is above 
the horizon ; (2) the longer or shorter time he 
is above the horizon. In summer he ascends 
to a much greater height above the horizon, and 
his rays thus reach the surface more directly 
than in winter, when his altitude is less and 
the rays fall more obliquely. In the former 
case, too, the rays before reaching the surface 
have to pass through a less thickness of denser 
atmosphere than in the latter case, and it 
is especially the lower denser layers of the 
atmosphere (or rather the vapours thereof) that 



290 Weather Science 

absorb much of the heat, allowing only a small 
portion to pass through. Moreover, the days 
in summer being longer than in winter, in our 
latitude, more heat is received from the sun 
during the day than is radiated away during the 
night, whilst the converse is the case in winter, 
when the days are shorter and the nights longer 
than in summer. 

Thus the general course of the seasons the 
cold of winter, the warmth of summer, and the 
intermediate conditions of spring and autumn- 
follows as a consequence of these variations in 
the amount of sunshine received. (In spring 
and autumn the sun's rays are less oblique than 
in winter, but not so direct as in summer, and 
the days are longer than the former, but shorter 
than those of the latter season, and so the tem- 
perature lies between those of the other periods.) 
In the same way in general the temperature of 
places nearer the Equator is higher than those 
further north and south ; the polar regions, where 
the sun is often absent for months at a time, and 
never at any time high in the sky, are the 
coldest parts of the earth. Thus the familiar 
geographical division of the earth's surface into 
five zones, the Torrid Zone, the North and South 
Temperate Zones, the Arctic and the Antarctic 
Zones. Within the Torrid Zone, 23 N. to 
23| S. latitude, which is bisected by the 
Equator, the sun is vertical at some time or 



The Five Zones 291 

other over every point, and its noonday altitude 
is never less than 43. It is above the horizon 
at all places for some time every day for a period 
never greater than thirteen and a half hours or less 
than ten and a half hours, so that day and night 
are never of very varying lengths. On 21st March 
and 23rd September the day and night are theo- 
retically everywhere of equal length, whence the 
name of " equinoxes " (Latin : equus, equal ; nox, 
night; i.e., night equal to day); at these times 
the sun is in the celestial Equator and is vertical 
at noon to all places on the terrestrial Equator. 
Practically, owing to the effect of the atmos- 
pheric refraction raising the sun's image above 
the horizon when it would otherwise be below, 
and causing twilight (a comparatively brief 
phenomenon in the tropical regions as com- 
pared to its duration in our latitudes), at these 
times the day is longer than the night. 

Here the greatest amount of sunlight is re- 
ceived, and consequently the name of Torrid 
Zone is given to this part of the earth which 
the ancient Greeks thought was too hot to be 
inhabited by human beings. 

On either side of the Torrid Zone there is 
(1) the North Temperate Zone; (2) the South 
Temperate Zone. Each of these zones ex- 
tends from 23! to 661 of latitude. Within 
these zones the sun is never vertical, but attains 
his greatest altitude above the horizon at noon 



292 Weather Science 

at the summer solstice. For the Northern 
Hemisphere this is on 21st June; for the 
Southern, 21st December. The limiting lati- 
tudes, 23| N. and 23^ S. respectively, bound- 
ing the Torrid and either temperate zones, are 
known by the names of the Tropics of Cancer 
and of Capricorn respectively, for at the time 
when the sun is said technically to enter the 
sign of Cancer, he is vertical at noon at latitude 
23^ N. (Tropic of Cancer) and begins his 
march southwards (Greek : r/jeVw, trepo, I turn), 
whilst on 21st December he is said to enter 
the sign Capricorn, and returns once more 
northward. Within the temperate zones the 
length of time the sun is above the horizon 
varies greatly according to the season of the 
year and the latitude of the place, but every- 
where he rises and sets at least once in the twenty- 
four hours. At either tropic the sun is vertical 
at noon for one day in the year, at other times 
the meridian altitude is less than this. At the 
northern and southern limits of the temperate 
zones, latitudes 66^ N. and 66^ S. respec- 
tively, which are correspondingly known as 
the Arctic and Antarctic Circles, the sun will 
theoretically be just on the horizon at the 
winter solstice of each hemisphere ; practically, 
owing to refraction, he will be slightly above 
that circle. Thus in latitude 66 N. on 
21st December the sun will not rise at all, 
and the length of the night will be twenty- 



The "Midnight" Sun 293 

four hours, and the same thing will be the 
case for latitude 66^ S. on 21st June. 
Conversely at the summer solstice of either 
hemisphere he will not set at all, but just 
skirt the horizon at " midnight," and the phenom- 
enon of the midnight sun may be seen. In 
higher latitudes, from 66^ N. to the North 
Pole and 66| S. to the South Pole, the Frigid 
Zones, there will be periods during which the 
sun will never set, but describe almost complete 
circles in the sky, having its greatest altitude at 
" noon " and least at " midnight," in the Northern 
Hemisphere, when due south and due north 
respectively, whilst at other times of the year 
the sun will never rise. Thus, in Lapland, 
Greenland, etc., there will be several weeks of 
perpetual day during the summer - time, and 
several weeks of perpetual night during the 
winter, whilst, for the rest of the year the 
sun will rise and set daily, but the length of 
the day will vary from a few minutes to 
nearly twenty-four hours, and conversely with 
the night. At the poles the sun will be 
above the horizon for about six months at a 
time (rather longer, through the effect of refrac- 
tion), its altitude varying with extreme slowness, 
so that for one day we may regard it as moving 
in a circle parallel to the horizon (parallel sphere). 
(See also chap, i.) 

Nevertheless, though thus continuously visible 
in the polar regions, the small altitude above the 



294 Weather Science 

horizon, never more than 23| at the summer 
solstice at either pole, or 47, its maximum value 
at noon on the Arctic Circle (21st June) or the 
Antarctic Circle (21st December), prevents the 
total amount of heat received by these regions 
even in summer from being so much as in lower 
latitudes. This theoretical and general distribu- 
tion of temperature throughout the globe is, 
however, only a rough approximation to the 
truth. Owing to the fact that seven-tenths 
of the earth's surface is covered by water, whose 
behaviour under the influence of heat is very 
different from that of the solid land, the actual 
temperature of different places is enormously 
influenced by their greater or less proximity to 
large masses of water. Water of all ordinary sub- 
stances * is the one whose specific heat is greatest, 
i.e., a greater amount of heat is required to raise 
a given mass of it through any temperature than 
is required to raise an equal mass of another 
substance through the same range of tempera- 
ture. Thus thirty times as much heat is required 
to raise a pound of water through 10, say from 
40 to 50, as is required to raise a pound of 
mercury from 40 to 50. In consequence of 
this, sudden increases or decreases in the amount 
of heat received are much slower in affecting the 
temperature of the sea than of the land. 

In summer the heat of the sun gradually 

1 A mixture of alcohol and water in certain proportions is said 
to have a somewhat greater specific heat than water ; but this 
is not a " common " substance. 



Contrasts of Climates 295 

warms up the water in the daytime, but more 
slowly than the land, owing to the greater 
specific heat of the former; whilst this heat is 
not radiated away again by the water during 
the night with anything like the same quickness 
as is the case with that received by the land, 
also in winter again the sea surface is not 
cooled so much as the land masses. Thus the 
neighbourhood of the sea keeps the land masses 
cooler than they otherwise would be in summer, 
and warmer than would otherwise be the case in 
winter. This is strikingly shown (though there 
are other causes also at work to be mentioned 
directly) in comparing the climates of the British 
Islands and Central Russia. The latitude of 
Edinburgh (55 55' N.) is almost the same as 
that of Moscow (55 45' N.), so that both 
places receive directly almost exactly the same 
total amount of heat from the sun annually. 
Yet how different are their climates ! The 
mean temperature of the former in January 
(the coldest month) is considerably above the 
freezing - point, at the latter far below that 
temperature, whilst in July (hottest month) the 
mean temperature at Edinburgh is under 60 F., 
at Moscow it is nearly 70. The proximity of 
our islands to the Atlantic Ocean prevents the 
winter temperature from falling so low, and 
hinders the summer temperature from rising so 
high, as with a region so remote from the ocean 
as Central Russia. In summer the land masses 



296 Weather Science 

impart some of their extra heat to the sea ; in 
winter the sea in its turn gives up some of its 
heat to the land. 

Another cause, and one of great importance 
to us as inhabitants of these islands, tending to 
the modification of temperature conditions, is the 
existence of ocean currents. (See also chap, x.) 
The intensely heated waters of the Gulf of 
Mexico give rise to the warm current of the Gulf 
Stream, which, flowing northwards and eastwards 
into the Atlantic, travels polewards almost as 
far as the latitude of the Arctic Circle, thereby 
raising the temperature of the British Islands 
and Western Europe generally. On the other 
hand, the cold current flowing from polar regions 
southwards renders the shores of Labrador, whose 
latitude is scarcely greater than that of Great 
Britain, ice-bound for most part of the year. 

By long continued observations of the tem- 
perature at different places throughout the world, 
mean values of this element for the whole year 
and for separate months have been obtained, 
and curves called isothermal lines (?<ro?, equal ; 
Oepw, heat) have been drawn. 

The idea of representing the distribution of 
temperature in this way is due to the great 
philosopher and traveller, Alexander von Hum- 
boldt, and charts showing the mean temperature 
of the whole globe for the year, as also for the 
months of January and July, are to be found 
in many works. A very detailed account of 



Course of the Isothermal, 50 F. 297 

the course of these curves is to be found in 
Scott's " Meteorology " (chap. xii.). Speaking in 
a very general sense, the main feature of most 
of these curves for the Northern Hemisphere is 
the remarkable fall in temperature in crossing 
the continental mass of Europe and " Asia 
eastwards, its rise again on approaching and 
crossing the Pacific Ocean, and fall once more 
in crossing North America." 

Taking one northern isothermal line as an 
example, say that of 50 F. or 10 C. (commencing 
with Europe), we find it crossing the centre of 
Ireland. Rising over the sea and dipping slightly 
in going over England, it enters the continent 
of Europe in North Germany; thence it runs 
south - eastwards, crosses Austria and South 
Russia, then touches the Caspian and Aral Seas 
at their northern extremities. It thence passes 
Lake Balkash, the Thian Shan Mountains, and 
Southern Mongolia, finally crossing the Japan Sea 
in latitude 40. Thus in crossing the Eurasian 
continent its latitude has fallen from 55 to 40. 
It next crosses the Pacific, its position becoming 
gradually more northerly, and enters America 
from Vancouver Island, latitude 50. Crossing 
the American continent it again dips southward, 
crossing the great lakes at about latitude 43, 
after which it rises again slightly, and enters the 
Atlantic in latitude 46, just south of Newfound- 
land. During its course across the ocean it works 
its way gradually more northwards till it comes 



298 Weather Science 

once more to the centre of Ireland (latitude 53), 
where we started with it. 

The isothermal lines for the Southern Hemi- 
sphere show similar dips when passing over 
continents, districts near the sea being gener- 
ally warmer than places of the same latitude 
further inland ; but owing to the much greater 
extent of water south of the Equator, the general 
course of these isotherms is more regular than 
for the opposite hemisphere. The isothermal 
line of 50 F. or 10 C. in the Southern 
Hemisphere runs almost along the parallel of 
42 S. latitude, dipping only slightly below this 
in crossing part of the Southern Pacific and 
Patagonia. Curiously enough, the isothermal 
line of 40 F., whose course is almost entirely 
over the ocean, only touching the southern 
portion of Patagonia is somewhat more irregular 
than that of 50 F., running from latitude 45 S. 
in the Indian Ocean south of Africa (longitude 
30 E.) to latitude 53 S. in the South Pacific, 
a range of about 8 in latitude, whilst the 
isotherm of 50 F. does not vary more than 
about 5 in latitude from 40 S. latitude in 
the (South) Indian Ocean to 45 C S. latitude 
in part of the Southern Pacific. But the 
range of the northern isothermals is vastly 
greater than this. By a comparison of the 
positions of the isothermal lines for the same 
temperature in the two hemispheres we find 
that on the whole, especially in high latitudes, 



Range in Temperature 299 

the Northern Hemisphere is colder than the 
Southern. 

There are, however, other considerations, 
besides the study of the course of the mean 
annual lines of temperature, which are of 
importance in giving us an idea as to the 
climate of any locality. Many places have the 
same or nearly the same annual mean tempera- 
ture, whose summer and winter extremes are 
very different. To take an extreme instance, 
the average range of temperature difference 
between winter and summer in these islands 
is only about 20, whilst in Verkhoyansk in 
Siberia, which has an unenviable notoriety as 
being the coldest place on the globe, this 
range amounts to no less than 100. The 
July temperature of this place is about 60 
(within 1 of that of Dublin) whilst the mean 
January temperature is 40, the freezing-point 
of mercury. At times the thermometer (of 
course a spirit one must be used here) reads 
much lower, 80 having been recorded, whilst 
in summer the temperature has risen to that 
! of blood heat. Thus between the extremes 
of -80 and + 96 we have a range of 176, 
nearly equal to that between the freezing and 
boiling points of water, the "mean" range 
being, as we have stated, about 120. Such a 
climate must cause a great strain on the 
powers of human endurance, and will hardly be 
recommended to the delicate invalid. We have 



300 Weather Science 

here the most extreme case of "continental" 
as opposed to "insular" climate. Even within 
our own islands, however, there is a considerable 
variety in respect to "insular" climate. The 
excessive rainfall of the west of Ireland and 
the lake district of Cumberland, as contrasted 
with the dryness of Leinster and the eastern 
counties of England (Lincolnshire, etc.), the 
warmth of Devon and Cornwall, and the cold 
of the northern and eastern districts, are well- 
known examples within a limited area, show- 
ing how much the general character of climate 
is affected by purely local considerations. 

The late Professor Dove, whose isothermal 
charts were amongst the earliest published, 
conceived the idea of representing the great 
contrasts prevailing between climates of places 
in the same latitude. He calculated for each 
month the normal temperature of each tenth 
degree of latitude, i.e., that which it would 
have if its actual temperature were uniformly 
arranged, and we take from Mr Scott's book 
Hann's corrected values of his numbers, in 
Fahrenheit degrees, for the mean of the year : 

Lat. 10 20 30 40 50 60 70" 80* 90 

N. 797 79-9 77*5 69-8 56'5 42'4 29'8 16'0 6'8 2'3 
S. 79*8 78-0 74-1 69-9 55*4 437 32'5 .j 

By comparison of the actually existing tempera- 
tures at various places with the mean values 
for the respective latitudes, Dove drew a series 
of curves representing what he called the 



Isabnormal Lines 301 

thermic anomaly " of each month, calling the 
curves by the name of isabnormals. Just as 
places on the same isothermal have the same 
temperature, so places on the same isabnormal 
have temperatures deviating by an equal 
number of degrees from that calculated for 
their latitude. We find from these curves that 
in January almost the whole of Asia is below 
the normal value, whilst in Eastern Siberia, 
round Verkhoyansk, the deviation is as much 
as 40 F. On the other hand, our own islands, 
as was to be expected, have temperatures much 
above the normal corresponding to their latitude, 
whilst in Iceland and the west of Scandinavia 
the excess amounts to as much as 40. Thus 
we see the negative anomaly of Eastern Siberia 
runs up to 40, whilst the positive anomaly of 
Iceland and the surrounding sea is as much 
in the opposite direction. The isabnormals for 
July (summer of Northern Hemisphere and 
winter of Southern) all show much less deviation 
from normal conditions, but here the heating 
of continental masses and the cooling effect of 
the ocean are still sufficiently manifest. Recent 
work on this subject by Eredia and Naccari 
with regard to the isabnormals of Italy, by 
Gheorghin on the Balkan Peninsula, etc., all 
bring out the important influence of the waters 
of the sea in modifying the conditions of 
temperature and pressure over land masses. 
Even the Mediterranean, small though its size is 

T 



3O2 Weather Science 

in comparison with the great oceans, produces 
a measurable effect. 

Of extreme temperatures experienced, though 
on the whole the equatorial regions are the 
hottest and the polar the coldest, yet actually 
on special occasions higher and lower tempera- 
tures than those experienced at the Equator 
and " nearest the Pole " are recorded elsewhere. 
In the centre of the Sahara the temperature 
occasionally mounts to 130 F., at Jacobabad, 
in the Sind desert, 120 has been recorded, both 
these districts being considerably north of the 
Equator, whilst in South Africa and Central 
Australia as great heat has been at times 
experienced. In London the highest summer 
shade temperature does not often exceed 95 F. 

On the other hand, as already stated, at 
Verkhoyansk, a village in Eastern Siberia 
(latitude 68 N.), "the coldest inhabited place 
on the globe," the thermometer has been known 
to descend to - 81 (1871), whilst during an Arctic 
expedition Captain Nares "once saw the ther- 
mometer descend to 84 F. below zero." The 
mean temperature of the Siberian town is, how- 
ever, higher than that of the Arctic Seas, though 
the winter cold is greater. 

It has been calculated by Ferrel that if 
the surface of the earth were entirely dry land 
and no transference of heat by ocean and atmos- 
pheric currents occurred, the temperature at the 
Equator would be about 131 F., that at the 



Equatorial and Polar Temperatures 303 

Pole - 108. As given already, Professor Hann's 
corrected values of Dove's figures are for these 
+80 and + 2 respectively for latitudes 
(Equator) and 90 (Pole). Thus the circula- 
tion produced by wind and ocean currents 
lowers the temperature of the equatorial region 
by about 51, and raises that of the polar regions 
by twice that amount, so rendering the whole 
earth habitable. It has been calculated that 
without an atmosphere the temperature even 
at the Equator would be -94 R, whilst at the 
Poles the thermometer would descend to - 328 
(Archibald) ; but these figures, though somewhat 
startling, can scarcely be said to have any mean- 
ing, except to suggest the possible extreme 
temperatures on an airless globe such as the 
moon is considered to be. 

In the most general sense, all meteorological 
phenomena depending ultimately on the action 
of the sun's rays upon the various constituents 
of the atmosphere, it is evident that a study 
of the laws of the conduction and radiation of 
this heat, and the motions (of matter) thereby 
following, is of the utmost importance in our 
subject. 

Thus the warm currents from the Equator 
towards the Poles, the colder polar currents 
flowing in to take their places, follow from the 
primary difference in the amount of heat received 
by each region respectively. The air at the 
Equator being heated more than that at the 



304 Weather Science 

Poles will expand more than the latter, and thus 
a change in the isobaric surfaces, or surfaces of 
equal pressure, will be produced, causing a flow 
towards the colder regions. Just as in a tank 
of water heated at one end the heated fluid will 
expand and tend to rise to a higher level than 
at the colder end, the difference of level will 
at once start a motion of the upper heated layers 
towards the further end of the tank, so, too, the 
air in the upper atmosphere will tend towards 
the polar regions. This flow will diminish the 
pressure on the surface, and so a current of 
air will come along the surface helping to 
restore the former equilibrium of pressure. 
Thus we get vertical ascending currents at the 
Equator, descending ones at the Poles, and 
intermediate regions of no motion. 

Owing to the earth's rotation on its axis, the 
equatorial air going polewards and the polar 
air going equatorwards are deviated, whilst the 
distribution of land and water, the annual 
(apparent) motion of the sun northwards and 
southwards, causing somewhat different regions 
to be the most highly heated, and countless 
other minor modifying causes, all come into 
play. 

We have already roughly sketched the dis- 
tribution of temperature on the land surfaces 
as determined primarily by the amount of sun- 
heat directly received, and modified by the 
action of winds and the greater or less proximity 



Range of Temperatures 305 

of the seas, with their various warm and cold 
currents ; it now remains to make a few remarks 
with regard to the temperatures prevailing over 
the sea itself. So far, though of recent years 
a good deal of information has been acquired 
as to temperatures below the surface, and a 
few observations at great depths have been 
made, most of our knowledge relates to that 
prevailing at the surface itself. 

In general, both the diurnal and the annual 
range of temperature at sea are smaller than 
for places on land, in corresponding latitudes. 
The difference between winter and summer 
temperatures are, however, easily noticeable for 
all localities outside the tropics, being, as was to 
be expected, more perceptible for inland seas and 
regions of the ocean nearest the land than for 
the open sea, far from influence of land masses. 

The range of temperature has its maximum 
and minimum values in general about one month 
later than on land ; on the latter, February and 
August are considered the coldest and the 
hottest months, whilst on the sea the "annual 
extremes of heat and cold occur in the months 
of March and September" (Maury). On land 
the solid parts of the earth receive from the 
sun more heat (during the summer of each 
hemisphere respectively) by day than they 
radiate by night, and thus even after the 
maximum amount of heat is received (summer 
solstice) the gain by day exceeds the loss 



306 Weather Science 

by night, and the temperature still rises for 
some time longer. So in our hemisphere the 
highest temperatures prevail in August and the 
latter part of July. At sea, owing to the 
greater specific heat of water over that of solid 
land, both rises and falls of temperature occur 
more gradually. Thus the waters continue to 
grow warmer even after the land temperatures 
have begun to fall, and the highest temperature 
comes in September, the lowest in March (in 
the Northern Hemisphere). 

On a few occasions, at places near the Equator, 
temperatures of 90 F. have been observed at 
the sea surface, but in general the sea tempera- 
ture seldom rises much above 80 F. in the 
Central Atlantic and Pacific, and the northern 
parts of the Indian Ocean. 

The northern isotherm of 80 during the 
month of March runs across the Atlantic from 
the Caribbean Sea towards Africa, with a slight 
southward trend, its position varying from 
latitude 15 N. to the Equator (in longitude 
22 W.), arriving near Cape Palmas, in Guinea, 
in about latitude 5 N. In September its 
course over the same ocean is much more 
sinuous. Starting from latitude 35 N., it runs 
westwards as far as longitude 40 W., after which 
it dips sharply southwards, reaches the Equator 
in longitude 26, and then runs back again to 
the South American coast, slightly to the south 
of the Equator. The southern isotherms of 80 






Isothermals of the Atlantic 307 

for September and March follow very different 
tracks. The former, starting from the African 
coast near the River Gambia, runs a short way 
westwards, then dips southward at longitude 
20 and returns to Africa in latitude 3 S., north 
of Loango in the Congo. The latter (southern 
isotherm of 80 for March), starting near Rio 
de Janeiro, runs with a northward trend towards 
Africa, making one or two southward dips on 
its way, and arrives in latitude 12 S., not far 
from Benguela. It attains its extreme northern 
position in the ocean (latitude 6 S.) at about 
2 E. longitude (almost due south of Dahomey). 
The isothermals for 70 run courses much more 
nearly parallel to one another. Though some- 
what closer together in position near the 
American coast than on the other side, the 
windings of the one follow very closely the 
convolutions of the other, so that by a shift 
of 20 in latitude, either may be obtained from 
the other. The March isotherm runs across the 
Atlantic (eastwards) with a general southerly 
trend, arriving off the African coast in latitude 
17 N. (not far from Cape Verde) ; the September 
isotherm touches the coast of Southern Portugal 
(latitude 38 N.). 

The influence of the Gulf Stream upon the 
course of the isotherms for lower temperatures 
is very strikingly shown. For instance, in the 
North Atlantic the northern branch of 50 
isothermal (in March), starting from just below 



308 Weather Science 

the latitude of New York (38 N.), runs east- 
wards and northwards past the shores of Nova 
Scotia and Newfoundland, thence towards Europe 
continually northwards till it reaches the latitude 
of 56 N. in longitude 17 W. ; after which it 
bends slightly southwards and reaches these 
islands near the south of Ireland. The course 
of the isotherm for 60 in September is not 
very dissimilar from that of the isotherm for 50 
in March, but it starts at a somewhat higher 
latitude (42 N.), not far from Cape Cod, and 
does not rise further than latitude 53 N. (in 
longitude 30 W.), arriving rather to the south- 
ward of the former's March position in Cornwall. 
In the Southern Hemisphere the isotherm of 
60 never deviates far from latitude 40 S. 
during its March course, and this very closely 
corresponds to the September isotherm of 50 
in the same hemisphere. The March and 
September southern isotherms of 60 do not 
vary much from parallelism during their course 
over the Atlantic, the mean position of the 
former being in latitude 40 S., that of the 
latter about latitude 35 S. Neither of these 
vary in latitude so much as 5 north or south 
of their mean position, a great contrast to the 
course of the isotherms for the corresponding 
temperatures in the North Atlantic, whose varia- 
tions are more than double that amount, and 
whose extreme limits of latitude differ by over 
20 (e.g., the March isotherm of 50 F. = 10 C. 



Pacific Isotherms 309 

is as far as 57 N. in longitude 17 W., and 
only 37 N. in longitude 73 W., whilst the 
variations of the September isotherm of 60 F. 
are of a slightly smaller range in latitude). 
The great northward trend of the North Atlantic 
isotherms strongly marks the influence of the 
warm "river in the ocean," whilst the absence 
of any similarly marked agency in the southern 
ones is shown by their closer coincidence with 
the latitude parallels. The smaller number of 
observations of sea temperature in the Pacific 
prevents any beyond very general inferences from 
being drawn from the course of the isotherms, 
but the influences of the seasonal changes of 
declination of the sun, as well as the special 
action of the ocean currents, are known to 
produce displacements corresponding to the 
former, and bendings in their course through the 
action of the latter. But the effect of currents 
is probably greater in the north temperate zone 
of the Atlantic than elsewhere, since the warm 
current of the Gulf Stream and the cold currents 
from the Arctic can more easily pass in and 
out, whereas in the Northern Pacific the warm 
current of the " Kuro Si wo " cannot easily pass 
out nor the colder waters of the Arctic enter 
through the narrow and shallow Behring Straits 
separating the Asiatic and American continents. 
It was at one time supposed that since the 
temperature of maximum density for fresh water 
is about 39-2 F. (4 C.) this temperature would 



3io Weather Science 

be found to prevail at great depths all over the 
world, but it has since been shown as a result 
of the " Challenger " expeditions as well as by 
others also, that a lower temperature than this 
is met with. " In all latitudes where soundings 
were made, a temperature of about 32 prevailed 
in the depths of the sea." 



Upper Air 311 



CHAPTER, XVII 

UPPER AIR BALLOON ASCENTS KITES ISOTHERMAL LAYER DIVISION 

OF AIR INTO TWO REGIONS, LOWER AND UPPER. 

OF recent years our knowledge of the conditions 
of the upper air has been increased by the results 
of balloon and kite ascents, and information has 
been obtained by means of self-recording instru- 
ments of the pressure and temperature prevailing 
up to a height of about 20 miles from the 
surface. Though it is true that we live at the 
" bottom of the vast atmospheric ocean " envelop- 
ing our globe, and writers have been fond of 
comparing our position to that of fishes at the 
bottom of the sea, yet there is but little real 
analogy in the two cases. 

At the bottom of the sea the density is but 
slightly greater than that at the top, owing to 
the slight compressibility of water ; no light can 
penetrate, and the most violent movements at 
the surface have no effect. On the other hand, 
the air, though reaching to an unknown height 
above the surface, decreases so rapidly in density 
upwards that at an altitude of 3 to 4 miles above 
the surface we have passed through half its mass, 
and the quantity above 20 miles from the surface 



312 Weather Science 

must be a very small fraction of the whole, and 
be of insignificant density. The highest clouds 
are not often formed at an altitude of more than 
5 or 6 miles, though occasionally a few are 
found at greater heights, and the quantity oi 
water vapour must be exceedingly small even 
at the height of the highest mountains. The 
experiments of Tyndall, and more recently those 
of other physicists in this country and on the 
continent, have shown that the oxygen and 
nitrogen of the air have almost no absorbent 
action upon the solar rays, the latter passing 
through them almost as readily as through a 
vacuum, but carbon dioxide and water vapour, 
on the other hand, exercise so considerable an 
absorption that the amount lost even when the 
sun is vertical is not less than 30 per cent, of the 
whole, and a much greater proportion when the 
sun is nearer the horizon. Langley estimated 
that an even greater amount than this is 
absorbed, the more refrangible shorter waves 
being absorbed to a far larger extent than the 
others, so that if we could see the sun as it 
really is, it would be of what we call a " bluish " 
colour. All of this absorbed heat, however, is 
not lost to us, for though a part is radiated 
outwards, another portion is sent downwards 
(from the clouds, etc.), and warms the surface 
of the ground and the lower air. 

More than a hundred years ago the brothers 
Montgolfier, by filling a silk or paper bag with 



Balloon Ascents 313 

hot air, caused the latter to ascend, and sent up 
sheep and fowls. Lichtenberg filled his balloon 
with hydrogen, and Pilatre de Rozier was the 
first person to make an ascent himself. His 
balloon was filled with hot air, not hydrogen, for 
it was found that the latter substance, though 
lighter, readily escaped through the pores of the 
paper. In later times coal gas has been generally 
used, from its cheapness and the ease with 
which it can be obtained. Many other ascents 
by Tissandier, Gay Lussac, Blanchard, Green, 
Welsh, etc., have been made since the time of 
the Montgolfiers, and the history of ballooning is 
full of terrible catastrophes and hair - breadth 
escapes, just as is the history of aeroplanes in our 
own day. The most interesting of those ascents 
from our point of view was the memorable series 
of balloon voyages made by Messrs Glaisher and 
Coxwell, who in 1862 reached an altitude which 
has never since been attained by human beings, 
rising to an elevation of nearly 37,000 feet, 7,000 
feet above the level of the highest mountain. 

The story is oft told how Mr Glaisher 
fainted, and his companion only managed to 
open the gas valve by means of his teeth, the 
cold being so great and the air pressure so low, 
estimated at equal to 7 inches of mercury (less 
than one quarter its value at the surface) whilst 
the temperature was far below the freezing-point 
of water, though the ascent was made in the 
summer-time. By this and other ascents there 



314 Weather Science 

was established an approximate rate of decrease 
of temperature upwards. A few of Mr Glaisher's 
averages are given below (Deschanel, " Physics," 
vol. ii.) : 

Rate of decrease of temperature 

upwards. 
Height above surface. With clear sky. With cloudy sky. 

From to 1,000 ft. 1 F. in 139 ft. 1 F. in 222 ft. 
to 10,000 1 F. in 288 1 F. in 331 
to 20,000 1 F. in 365 1 F. in 468 



This table shows also the effect of clouds in 
diminishing this rate of fall, it being necessary to 
go about one-third higher in a cloudy atmosphere 
to produce the same fall of temperature as when 
the sky is clear. These merely approximate 
numbers (for Great Britain in the daytime 
during the summer) differ in different parts of 
the earth, and it is not uncommon to find 
alternations of increase and decrease at certain 
times. (The rate of decrease of temperature in 
going above sea-level in mountainous regions of 
this country, is rather less than these values, 
being about 1 F. for every 300 feet.) Since 
that time M. Hermite, a French observer, sent 
up a balloon with recording instruments, which 
rose to a height even greater than that attained 
by Messrs Glaisher and Coxwell. It ascended to 
a point where the pressure was only 4*1 inches of 
mercury and the temperature 104 F., having 
probably risen to a height of 10 miles in the air. 
From the results of this ascent it would seem 
that the temperature of the air decreases pretty 



Isothermal Layer 315 

regularly above 12,000 feet at the rate of 1 for 
every 330 feet rise in the air. 

But more recent observations recorded by the 
self-registering instruments taken up by means 
of kites (at the Observatory at Blue Hill, near 
Boston, Mass., U.S.A.) have shown that this 
decrease does not extend indefinitely upwards, 
but comes abruptly to an end after a certain 
altitude is attained. The most remarkable dis- 
covery has been made of the existence of an 
" isothermal layer," whose temperature varies 
very little over a wide vertical range. 

Up to a certain height varying somewhat with 
the time, pressure distribution, and latitude of 
the region, the temperature falls to a minimum 
value. Then the diminution abruptly ceases 
and for a great height the temperature is almost 
constant, "showing on the average a slight 
tendency to increase" (Gold). The average 
height of this isothermal layer for places in 
Europe, near latitude 50, is given at 11 kilo- 
metres (about 7 miles), and the temperature 
as -55 C. ( = -67 F.). The annual variation 
in this height is about 2'5 kilometres, and in 
temperature 10 C. (1 miles and 18 F.). On 
the whole the height is greatest in autumn, 
and its temperature greatest in summer. The 
value for Europe is about 10 kilometres (6 miles) 
over regions of low pressure, and 12*5 kilometres 
(8 miles) over places of high pressure. Thus the 
results of Hermite, given above, must have been 



CHAPTER XVIII 

PRACTICAL APPLICATIONS " SIGNS " OF BAIN THE LEECH SEASONAL 

CHANGES MIGRATION OP BIRDS PLANT ff PHENOLOGY " 

FLOWERING OF PLANTS FALL OF LEAVES SEASONAL PREVALENCE 

OF DISEASES SALUBRITY OF PARTICULAR REGIONS OZONE. 

AMONGST miscellaneous phenomena directly de- 
pendent on meteorological conditions, the first 
appearance of birds in the spring, the dates of 
budding of trees and flowers, the migration of 
birds and the "falling of the leaf" in autumn, 
the variation in the times of harvest (corn, 
vintage, etc.), have been noticed by various 
observers from comparatively early dates. From 
the recorded dates of the vintage ("grape harvest") 
in Central Europe for the last five hundred 
years, Dr Bruckner was enabled to arrive at im- 
portant inferences as to the succession of warm 
and cold periods during that time (chap. xv.). 
On the amount of rainfall in greater or less 
quantity throughout the year depends the suita- 
bility or otherwise of different districts for 
various crops. The relation of climate to health 
is another important branch of applied meteor- 
ology, and has always attracted a good deal of 
attention, though it is only of recent years that 



Seasons for Epidemics 319 

the subject has been treated in a careful scientific 
manner. Statistics dealing with the seasonal 
variations of epidemic diseases are of interest 
to the medical practitioner, for it is fairly well 
known how certain complaints are more prev- 
alent during certain seasons of the year than 
at other times ; for instance, scarlet fever and 
enteric give the maximum mortality during the 
autumn months, October and November, whilst 
these effects are at a minimum in the spring, 
March to May (Archibald). The peculiar real or 
alleged beneficial effects of ozone are also points 
about which there is a good deal of imagina- 
tion, but little real knowledge. 

Bruckner's work on the oscillations in mean 
temperature and rainfall, has already been 
mentioned (chap, xv.), as also his attempt to 
deduce an average period of about thirty-five to 
forty years for the recurrence of similar weather 
conditions. 

Throughout the literature of almost every 
nation we meet with numerous proverbs relating 
to the appearance or disappearance of birds 
and insects, and the blossoming, fruition, etc., 
of plants in greater or less abundance has been 
taken as a sign of the severity or mildness of 
the coming winter, whilst inferences are also 
drawn from their earlier or later appearance. 

Many signs of coming rain, some real and 
some imaginary, are collected from the behaviour 
of animals, birds, and plants. The remarks of 



320 Weather Science 

Mr Inwards (" Weather Lore ") are so appro- 
priate in this connection that we will now 
proceed to quote them verbatim: 

"The observations of naturalists, shepherds, 
herdsmen, and others, who have been brought 
much into contact with animals, have proved 
most clearly that these creatures are cognisant 
of approaching changes in the state of the air 
long before we know of their coming by other 
signs. To many kinds of animals, birds, and 
insects the weather is of so much more import- 
ance than to us, that it would be wonderful if 
nature had not provided them with a more 
keenly prophetic instinct in this respect." 

As signs of coming rain, the uneasiness of 
domestic animals, the low flight and short ex- 
cursions of birds and insects, the behaviour of 
bees, etc., are well known (see also chap. xiv.). 

The leech would appear to be an especially 
sensitive subject, from whose various behaviour 
a variety of influences may be drawn. Thus 
we are told that he is always agitated by 
changes of weather. He moves about very 
briskly before high wind. A leech confined in 
a bottle of water creeps to the top of the bottle, 
but soon sinks previous to slight rain or snow, 
but if the rain is likely to last longer, he 
remains a longer time at the surface. Before 
the approach of thunder he starts about in 
an agitated and convulsive manner (Inwards). 
Other inferences from the flight and movement 



Migrations of Birds 321 

of rooks are known in Scotland (and this country 
also). Thus their low flight and frequent return 
to the rookery are signs of coming rain, sitting 
on dykes and palings in rows are signs of wind, 
whilst their high flight, on going home to roost, 
indicates that the next day will be fine. A 
Devonshire proverb says : " If rooks stay at 
home, or return in the middle of the day, it 
will rain ; if they go far abroad, it will be fair." 

As signs of a seasonal character, the early 
migration of the summer birds is taken to 
indicate a cold winter; the appearance of the 
robin, woodcock, etc., tells the same story, 
whilst the martin is supposed to presage milder 
conditions. 

Of still more immediate connection with 
meteorological conditions is the study of plant 
"Phenology" (from Greek <palvco, more correctly, 
middle, Qaivopai, I appear), or the study of the 
conditions governing their flowering, fruition, 
and abundance, or otherwise. Plants are much 
more closely influenced by conditions of tempera- 
ture and moisture than animals. The practical 
interest of this study, as enabling inferences to 
be made as to the proper time for planting crops 
in various situations, the probable yield, etc., 
matters concerning which at present personal 
experience of former results is our only guide, is 
evident. By a careful study of observations of 
the times of occurrence of the various phenomena 
it is not too much to hope that in time valuable 






322 Weather Science 

information, based on the accumulated experience 
of many, will enable better results to be obtained 
in the future than has been the case in the past. 
A division of the spring time into five periods, 
each characterised by the blossoming of certain 
plants, has been made by Von Reichenau for the 
middle Rhine region of Germany. 

The first period, when the following plants 
blossom daphne, hepatica, pulsatilla. 

The second period, for the blooming of the 
stone fruits apricots, peaches, cherries, and 
plums. 

The third period, for the seed fruits pears 
and apples. 

The fourth period, " full spring." 

The fifth period, the end of spring, char- 
acterised by the flowering of rose, linden, and 
vine (Waldo). 

The oscillations in temperature of different 
years are well shown by the range in the date 
of the first appearance of bloom on the various 
species of plants, there being a range of thirty- 
nine days in the case of the elm and apricot, 
but only nineteen days for the vine. 

This subject has been rather a favourite one 
with naturalists, and attempts have been made 
to draw up a calendar of " Indications of Spring," 
the times of earliest and latest flowering. 

Mr Inwards, in his " Weather Lore," gives a 
long list (from Hone's "Everyday Book") of 
common plants, and the times at which they 



Flowering of Plants 323 

ought to be in full flower, from which we 
extract the following, in order of date. Beginning 
with the groundsel, which should appear on 
January 2, the dead nettle and anemone follow, 
and by the end of the month the double daisy 
should be in flower. In February the snow- 
drop first appears, soon followed by the primrose, 
hyacinth, and crocus, whilst towards the end 
of the month the peach is in flower. In March 
we have first the chickweed, this is followed by 
the violet in the middle of the month, and the 
marigold and oxlip at the end. April opens 
with the white violet, the wood anemone follows, 
then the dandelion, and later on the harebell, 
the cowslip coming last. The rhododendron 
should be in flower at the beginning of May, 
then the lily of the valley, the poppy, monks- 
hood, and horse chestnut. The lilac follows 
towards the end of the month, and the buttercup 
at the end. In June, " the month of roses," we 
have, of course, the various varieties of the 
"queen of flowers," also the pimpernel and 
pink at the beginning, the horned poppy in 
the middle, the St John's wort, sweet-william, 
and cornflower towards the end of the month. 
July opens with the agrimony and the white lily ; 
the nasturtium and snap-dragon follow, the con- 
volvulus and sweet-pea come later, and towards 
the end of the month the field camomile makes 
its appearance. The yellow tiger lily opens early 
in August, with hollyhocks and bluebells; the 



324 Weather Science 

common balsam appears in the middle of the 
month, then the sunflower, with the golden rod 
and yellow hollyhock at the end. 

Not many new flowers make their appearance 
in September ; mushrooms are plentiful, spring- 
ing up after every shower; the autumn crocus 
and the passion flower are in bloom. In October 
the soap wort, camomile and chrysanthemum first 
make their appearance, then later we have the 
holly, yarrow, and ten -leaved sunflower. In 
November the laurustinum is given for the 1st, 
the sweet butterbur for the 25th. For December 
we have the Barbadoes gooseberry and the arbor 
vitce in the first few days, whilst the cedar of 
Lebanon and the purple heath come at Christmas. 

A converse phenomenon to flowering, i.e., the 
fall of the leaf, is subject to considerable local 
and temporal variation. In some years the trees 
will begin to drop their leaves early and quickly, 
whilst the process will last for some weeks in 
other years. Moreover, the sequence of fall- 
ing varies; for instance, as indicated by the 
proverb : 

" If the oak's before the ash 
Then you'll only get a splash ; 
But if the ash precedes the oak 
Then you may expect a soak." 

The amount of rainfall thus being supposed 
indicated by the relative times of fall of the two 
different kinds of trees. The greater or less 
abundance of fruit has also been thought by 



Fruit 325 

some to have a bearing on the probable char- 
acter of the coming winter. Thus we are told : 
" If the oak bears much mast, it foreshows a 
long and hard winter " ; and we have also : 

" Many hips and haws, 
Many frosts and snaws." 

The abundance of fruit being thus supposed 
a provision for the birds during a long, hard, 
and trying winter. The thin or thick skin of 
the onion is thus apostrophised: 

" Onion's skin very thin, 
Mild winter coming in ; 
Onion's skin thick and tough, 
Coming winter cold and rough." 

Amongst many other prognostics of severe 
weather we have such as the following : " If 
many white -thorn blossoms or dog roses are 
seen, expect a severe winter" (Inwards). But 
this, we are told, is a fallacy. It certainly pre- 
supposes a wonderful degree of prescience on 
the part of the vegetable kingdom ! " If in the 
fall of the leaves in October many wither on 
the boughs and hang there, it betokens a 
frosty winter and much snow." And again we 
learn : 

" If on the trees the leaves still hold 
The coming winter will be cold." 

As already stated (chap, xv.), by his ex- 
amination of the records of the times of the 



326 Weather Science 

"grape harvest" in France, Switzerland, and 
the Rhine district for several hundred years 
past, Dr Bruckner has been able to confirm 
his results as to a period of general oscillation 
of mean temperature of about thirty-four or 
thirty-five years or so, and from lists of severe 
winters given by Pilgram and others, he has 
extended his researches as far back as A.D. 800, 
a period of over one thousand years. 

A comparison of the average quinquennial or 
" lustral " periods (" of five years "), with the 
records for rainfall and temperature, shows that 
"in general" the earlier period of harvest is 
identical with that of high temperature and 
small rainfall, whilst conversely the times of 
late harvest and low temperature occur together 
(Waldo). 

An important branch of applied meteorology, 
the collection of statistics bearing on the relation 
of climate to disease, and the greater or less 
healthfullness of certain localities, has attracted 
considerable attention at different times, and 
though much remains to be known, some con- 
clusions of great interest have been arrived at. 
It has long been known that certain epidemics 
are more prevalent at one season of the year 
than another, and we have already referred to the 
fact that whilst enteric fever is most prevalent 
and fatal during the autumn, on the other 
hand smallpox is less prevalent at that time, 
but most fatal later in the winter. Influ- 



Climate 327 

enza, heard much more of in recent years than 
formerly, "is evidently propagated through the 
air." The air of cities usually contains far more 
dust particles than that of the country, and is 
less healthful than the latter (with exceptions), 
for in certain districts remote from towns decay- 
ing vegetable growths may also occasion malarial 
fevers, as is the case with some regions of Italy, 
to say nothing of tropical districts, "mangrove 
swamps," etc. The difference between extreme 
summer and winter temperatures is a far more 
important factor affecting the health of the 
inhabitants of a district than the mean annual 
value of the temperature. Thus the mean 
annual temperature of New York is not far 
from that of London, but it is well known that 
the heat of summer is much greater, and the 
cold of winter more severe, in the former than 
the latter, the differences being those character- 
istic of "continental" and "insular" climates 
respectively. 

The smaller the annual range of temperature, 
other things being equal, the more salubrious 
the climate, and in general the proximity of 
the sea is most effective in equalising thermal 
conditions. 

There are a number of factors sometimes 
enumerated as affecting the climate of any 
district: Firstly, the latitude; the nearer the 
Equator, the hotter the climate. Secondly, 
situation in Northern or Southern Hemisphere. 



328 Weather Science 

Places in the Southern Hemisphere, owing to 
the great preponderance of water, are subject 
to much smaller variations of temperature than 
those of the same latitude in the opposite hemi- 
sphere. Thirdly, easterly districts on either 
continent (e.g., the Eastern United States and 
China) are subject to greater variations of tem- 
perature than westerly ones (e.g., California, 
France). 

Amongst other causes, elevation or altitude 
above sea-level is very important. The inland 
plateau of Tibet, notwithstanding its low lati- 
tude, is subject to very severe weather in winter. 
The neighbourhood of mountain ranges, situation 
with respect to winds, the influence of hot or 
cold currents, etc., all exercise important in- 
fluences, many of which have been dealt with 
in the course of this work. 

The greater or less amount of ozone and 
its real or imaginary influence upon the healthi- 
ness of a district are matters upon which much 
speculation has been indulged in, but very little 
is really known. Ozone appears to be formed 
during thunderstorms by the passage of the 
electric spark through the air, and is artificially 
produced by passing a series of discharges 
through oxygen, its name (from the Greek ofo>, 
I smell) being given to it from the peculiar 
smell often noticed during the working of 
electrical machines. Chemically, it appears to 
be compressed oxygen, the active element in 



Nature of Ozone 329 

the air. If a quantity of oxygen be submitted 
to the silent discharge, it will be found to 
contract in volume, but on being heated to 
about 500 F. it recovers its original bulk 
(allowing for the increase of volume due to the 
elevated temperature). Chemically, ozone is 
regarded as oxygen in which three molecules 
occupy the space of two in common oxygen, and 
the third molecule is held loosely in combination 
with the other two, so that it is readily given 
off, and oxidises combustible matter, destroying 
decomposing animal and vegetable substances 
by uniting with them, thus acting as a purify- 
ing agent. It appears to be more abundant 
near the sea coast than in inland districts, and 
is found in greater abundance in spring than 
at other times of the year. A smaller amount 
is found in summer, less still in autumn, and 
least of all in winter. 

Other observations, however, seem to show 
that the amount of ozone in air unexposed to 
the influence of the land is very nearly constant 
(Thorpe). The air just over marshes or malarial 
districts contains little or no ozone, but in the 
heights above such places some may often be 
found. No ozone can be detected in cities or 
in the air of dwelling-rooms. Though obtained 
as we have stated above, no method has yet 
been devised whereby it can be isolated; it is 
always found mixed with a much larger quantity 
of common air or oxygen, and even when most 



330 Weather Science 

concentrated does not form more than about 
8 per cent, of the mixture. 

Paper moistened with a solution of iodide of 
potassium and starch, and exposed to ozonised 
air, is rapidly turned a brilliant blue colour ; this 
is one of the most delicate tests for its detection, 
but is not an infallible one, since other sub- 
stances besides ozone affect the starch in the 
same way. Whilst a small quantity may be 
beneficial, on the other hand, however, there is 
little doubt that an excessive amount of ozone 
in the air is injurious to health. 



INDEX 



BERCROMBY, Hon. R., 17,20, 103, 

106, 117, 126, 132, 138, 227, 231, 

281 

ccademia del Citnento (barometer), 

48 

plhas current, 193 

ir and water, oceans of, contrasted, 

311 ; mixture of saturated and 

unsaturated, 163 

Aitken, J., 141, 143, 170, 180 

Almagest of Ptolemy, 15 

Anemometer, various forms, Robin- 
son's, Dines', Lind's, Wild's, etc., 
96, 97, 251 

Annual motion of sun (earth), 24 

Anticyclones, 22, 109, 125 et seq. 

the Atlantic, 231, 237 

Apjohn, Dr, 76 

Archibald, J. ; on cyclones, 105 ; 
speed of clouds, 142 ; probable 
temperature without an atmos- 
phere, 303 ; thunder clouds, 212 ; 
wind velocities, 113 

Aristophanes ("The Clouds"), 145 

Aristotle, his division of the atmos- 
phere into three regions, 15 

Atmosphere, the, constitution, 31, 
39 ; extent, 38 ; isothermal layer, 
315 ; new gases in, 40 ; pressure, 
32, 161 ; refraction, 33 ; tempera- 
ture, 42, 303 ; upper, 38, 311 ; 
weight, 39 

August, Dr (the Psychrometer), 76 

Aurora Borealis and Australia, 216 ; 
artificial, 224 

BACON, Lord, on cycles of weather, 

272 

Ball lightning, 207 
Balloon ascents, 312 ; highest, 313 
Banbury, shepherd of (weather 

prophet), 146 

Barnaoul, high barometer at, 65 
Barometer, as a "weather glass," 18, 

62 ; its history, 46 et seq. ; its con- 



struction, 48 ; glycerine, 48, 244 ; 

Fortin's (mercurial) form, 50 ; the 

vernier, 51 ; syphon, 52 ; fishery, 

18, 53 ; aneroid, 53 ; wheel, 55 ; 

self - recording (Barographs), 55 ; 

corrections to reading, 57 ; range 

of, 63 ; prognostics, 103 et seq., 

Prof. Dove's rules, 66 
Beaufort, Admiral, his wind scale, 

149, 159 
Bebber, Dr (on course of cyclones), 

123 

Bee, a fine weather insect, 270 
Bezold, W. von, 14 
Bible, cloud prognostic, 265 
Blandford, on sea and land breezes, 

159 
Blizzards and "barbers," cold winds 

of North America, 156, 157 
Blue Hill, (U.S.A.) observations, 113, 

315 
Boiling-point, varies with pressure, 

74 

Bora, cold wind, 157 
British rainfall, 89 
Bruckner Prof., his investigations, 

273 et seq. 
Buchan, J., 120, 286 ; warm and cold 

periods, 227 
Buist, Dr, quoted, 40 
Buy Ballot, his law, 20, 117, 151 



CALCUTTA, daily range of barometer 

at, 63 
Cape of Storms, old name for Cape 

of Good Hope, 194 
Carbon dioxide, its varying amount 

in the air, 39 

Cavallo, P., on a fireball, 207 
Celsius, 73 

Centigrade thermometer, 73 
Chambers, F., 279 

G. F., 67, 230, 257 

Charts, synoptic, 19, 105 



331 



332 



Index 



Cherrapunji, excessive rainfall at, 98 

Chenook, warm wind of Canada, 156 

Circulation, in upper and lower 
currents of cyclones and anti- 
cyclones, 112 

Cirro cumulus clouds, 134 

stratus clouds, 137 

Cirrus clouds, 131, 133 

haze (cirro nebula), 138 

Clayton, cloud observations at Blue 
Hill, U.S.A., 113 

Climates, insular and continental 
denned, 30 ; differences for some 
latitudes, 295 ; influenced by ocean 
currents, 296 ; extremes of, 327 

Cloudiness, amount of, how esti- 
mated, 141 

Clouds, kinds of, 131 et seq. ; height 
and motion, 139, 142; speed at 
different levels, 113, 142 317; 
"Hats," 147; prognostics, 144; 
Biblical references to, 265 

Col, the, 109 

Coleridge, S. T., "Ancient Mariner" 
quoted, 237 

Columbus, Christopher ; discovers 
magnetic "variation," 221; his 
son, on "St Elmo's" fire, 215 

Compass, points of, 149 

11 Cone and Drum," 256 

Convection of heat, 182 

Coronas, meteorological phenomena, 

Coxwell, see Glashier. 

Croll, Dr J., on heat conveyed by 
Gulf Stream, 186 

Cumulus clouds, 185, 268 

Currents ; air, equatorial and polar, 
151, 296 ; ocean, 181 et seq 

Cyclones, 21, 105 et seq. ; speed of 
described, 105 et seq., 114 ; definite 
course of, 122, 124 ; tropical, 106, 
128, et seq.; "eye" of, 121 ; air 
movements in, 124 



B'ABBADIB, on Abyssinian thunder- 
storms, 214 

Dalibard, draws sparks from clouds, 
204 

Dampier, W., on "sea breezes," 

De Dominis, Antonio (on rainbow), 

171 

De L'Isle, thermometer scale, 73 
De Saussure ; hair hygrometer, 77 ; 

" electric hum," 215 
"Desolate " region of Pacific, 191 
Dews, 37, 167 



Dines, W. H., on atmospheric tides, 
64 ; anemometer, 97 ; on dew, 169 

Districts, (meteorological) of British 
Isles, 257 

"Doldrums, "the, 225, 237 

Dove, Prof., wind and barometer, 
66 ; theory of hail, 165 ; tempera- 
ture at different latitudes, 300 

Dry and rainy seasons, 100 

Dust (necessary for fog), 141, 170 ; 
Aitken on, 180 



EARTH, size and shape, 22 ; motions, 

23 ; axis inclined to orbit, 24 ; 

zones, 27, 291 
Ebell (and Kurz), on ionisation of 

atmosphere, 200 
Electrometer, Quadrant (Kelvin's), 

202 
Epidemics, seasonal recurrence of, 

326 
Equatorial and polar currents, 151, 

" Equinoctial " gales, 227 

Equinoxes, the, 25, 291 

Eredia, P., 228, 301 

Eurydice. H.M.S., capsizing through 

"line squall," 108 
Everett, Prof. J. D., 199 
"Eye " of a cyclone, 121 



FAHRENHEIT, his scale, 72 

Ferrel, Prof. W., on cyclones, 120, 
236 ; on deviation due to earth's 
rotation, 152 ; on temperatures at 
Equator and Poles, 302 

Fitzroy, Admiral ; fishery barometer, 
53 ; cloud prognostics, 144 ; on 
colours of the sky, 269 ; cone and 
drum (storm warnings), 256 

Flammarion, C. (on the Mistral), 157, 
158; on hailstones, 166; on "St 
Elmo's fire," 215 ; on heat from 
moon, 282 

Florentine thermometer, 72 

Flowering of plants for each month, 
322 

Fogs, 140, 169 

Fohn wind of Switzerland, 155 

Fracto-cumulus or scud, 138 

Franklin, Benjamin, on Gulf Stream, 
189; on identity of atmospheric 
and fractional electricity, 204 ; on 
lightning conductors, 209 

Frauenhofer, J., artificial corona, 
176 

Fwwrolles of Tuscany, 40 



Index 



333 



GALILEO, and the barometer, 45 ; the 
thermometer, 71 ; discovers laws 
of motion, 285 

Glaisher, J., on hygrometrie tables, 
77 ; thermometer screen, 83 ; wind 
directions in Great Britain, 155 ; 
(Coxwell, and), balloon ascents, 313 
Glycerine barometer, 48, 245 
Gold, E., on isothermal layer, 315 
Grape-harvest (Vintage), Bruckner 

on times of, 276, 326 
"Graupel" or soft hail, 166 
Gravity, value at different latitudes, 

61 " 

Gulf Stream; its course, 184; 
influence on storms, 188 ; theories 
of cause, 189 ; influence on position 
of isothermals (heating effects), 189 

HADLET, J., 152 

Hail, 165, 171 

Halley, E., 72 

Halos, solar and lunar, 174, 263 

Hann, Dr J., on rainfall, 99 ; on 
winds, 154; on distribution of 
pressure, 275 ; on normal tempera- 
tures for different latitudes, 300 

Harmattan, hot east wind of West 
Africa, 156 

Harris, Sir W. Snow, his electro- 
meter, 203 

Haughton, Rev. Prof., on Gulf 
Stream, heating effects, 187 

Heat, modes of transference, 181 

Hepworth, Capt. M. W. C., 160 

Hermite, balloon ascent records, 314 

Herschel, Sir J., 39, 63, 84, 283, 288 

, Sir W., 283 

Homogeneous atmosphere, its height, 
32 

Hot and cold spells, 228 et sey. ; 
periods (Bruckner), 273 

Howard, Luke, his cloud nomen- 
clature, 131 et seq. ; cloud prog- 
nostics, 266 

Humboldt, Alexander von, 169, 296 ; 
his current, 190 

Hydrogen, its absence in the free 
state (?), 40 

Hygrodeik (illustration), 88 

Hygrometer or Psychrometer, wet 
and dry bulb, 75; Regnault's, 77; De 
Saussure's (hair Hygrometer), 77 

Hygroscopes and Hygrometers, 77 

Hypsometer (boiling - point ther- 
mometer), 75 

ICE PERIODS (Bruckner on), 276 
" Indian Summer," 229 



Inwards, R. ("Weather Lore," etc.), 

103, 140, 224, 260, 266, 283, 320, 

322, 325, etc. 
lonisation of the atmosphere, Ebell 

and Kurz on, 200 
Isabnormal lines, 301 
Isobars, denned, 19 ; straight, 109 
Isoclinic and isogonal lines, 222 
Isothermal layer, the, 315 
lines, 19 ; map, (British), 286 ; 

course of some, 297 ; across oceans, 

306 

JAMAICA, " sea breeze of," 159 
Jevons, W. S., 281 

KAEMTZ, J. , on the Mistral, 157 
Kelvin, Lord, Quadrant electrometer, 

202 
Kew Observatory, 243 ; testing of 

instruments, 58 
Khamsin, wind of Egypt, 156 
Kites, flying of, 315 
Krakatoa, eruption of, 179 
" Kuro-siwo," ocean current, 192, 309 
Kurz, see Ebell and Kurz 

LAKES (Bruckner), on variation of 

water level in, 274 
Lammas floods, 229 
Lamont, terrestrial magnetism and 

sun-spots, 278 
Land and sea breezes, 158 
Langdon, Mr, on atmospheric tides, 

64 
Langley, Prof. S. P., on true colour 

of sun, 179, 312 
Laplace and Napoleon, 17 
Leaves, fall of, 324 
Leech, behaviour of the, 320 
Lemstrom, Prof., produces artificial 

aurora, 224 

Leveche, wind of Spain, 156 
Ley, Rev. Clement, 138 
Lightning, kinds of, 206, conductors, 

209 

Lind, his anemometer, 97 
"Line squall, "108, 128 
LinnjBus, inventor of Centigrade 

scale, 73 
Lockyer, Sir J. N., on rainfall and 

sun-spots, 279 

Lodge, Sir 0. J., on lightnine, 209 
Longfellow, H. W., Wreck of the 

Hesperus, quoted, 263 
Looming, phenomena of, 178 
Loomis, Prof. E., on shape of 

cyclones, 124 



334 



Index 



MAGNETIC and true bearing, 148, 223 
storms, 217, 278 ; needle, 

its movements, 219 ; poles and 

equator, 222 ; movements of 

needle and sun-spots, 276 
Marie Davy on "the Mistral," 157 
Markree, rain gauges, 247 ; high 

barometer readings, 65 
Mascart, on electrification at Paris, 

200 

Maunder, E. W., 119, 280 
Maury, Commander, M. F., 184, 188, 

305 et sea. 
Maxwell, Prof. J. Clerk, on lightning 

conductors, 209 

May, the three cold days of, 228 
Mean latitude (45), 61 

sea-level, 61 

Meldrum, Dr J., on movements of 

cyclonic storms, 119 ; cyclones and 

sun-spots, 279 
Mercury, for barometer, 47 ; for 

thermometer, 72 et seq. 
Meteoric dust in the air, 39 
Meteorological districts, division of 

British Isles into, 255, 257 
office, British, 256 ; daily and 

weekly weather reports, 254, 259 ; 

Society, Royal, Glaisher's winds, 155 
Meteorology, science of observation, 

14, 45 ; average, 104 ; celestial, 

(?)44 

Meteors, celestial and not terres- 
trial objects, 15 
Metric and British measures, 260, 

(note) 

Midnight Sun, 293 
Mill, Dr H. R., 89, 198, 286 
Mirage, the, 159, 177 
Mist, 141, 171 
Mistral, wind, 157 
Mock suns (parhelia) and mock 

moons (paraselence), 174 
Mohn, Prof., ( " Meteorologie " ), 67 
Monsoons, 153 
Montgolfier, Brothers, balloon ascents , 

312 

Moon, and weather, supposed con- 
nection, 261, 264, 281 ; atmospheric 

tides, 64, 183 
Mozambique current, 193 

NAPOLEON and Laplace, 17 
Nature, magazine, 205, 279 
Negretti and Zambra, maximum 

thermometers, 78 
Newton, Sir I., explains rainbow, 

171 ; discovers law of gravitation, 

285 



Nimbus, the rain cloud, 137 ; Aber- 

cromby on, 138 

Nipher, shielded rain gauge, 92 
Nordenskiold, on the Aurora, 216 
Nortes, winds at Panama, 150 
Northert, of New Mexico, 150 
North - westers, hot winds of New 

Zealand, 155 

Northern and Southern Hemispheres, 
difference of temperature in corre- 
sponding latitudes, 299 



OBSERVATORIES, first, second, and 
third order stations, 240 ; Green- 
wich, 241 ; Kew, 243 ; mountain, 
244 

Oceans of air and water contrasted, 
311 

Ochtertyre, low barometer reading 
at, 65 

Ozone, its production, 328 ; test for, 
330 



PAMPERO, storm of Argentina, 128 

Parhelia and paraselence, 174 

Pascal, Blaise, 47, 162 

Paulsen, Dr, height of aurora above 
surface of ground, 224 

Peruvian or Humboldt current, 190 

Phillips^ Prof., his thermometer, 
78 

Pimpernel, ploughman's weather- 
glass, 271 

Planets, weather conditions on (?), 44 

Plant phenology, 321 

Plants as weather prophets, 271 

Poisson, his theorem on deviation due 
to earth's rotation, 152 

Polar currents, 194, 296 

Poles, phenomena at, 29 

Pressure, atmosphere variation 
throughout Globe, 154 ; at 
different heights (note), 69 

Provence, land of the Mistral, 158 

Psychrometer, see Hygrometer 

Ptolemy, Claudius, his "Almagest," 
15 

QUETELET, P., on atmospheric 
electricity, 199, 202 



EADIATION fogs, 171 

"Radiation," weather (Abercromby), 

126 

Rain gauge, 90 ; hour of reading, 250 
Rainbow, 171 ; lunar, 173 



Index 



335 



Rainfall, British (volume), 89 
Rainfall, measurement of, 89 et seq. ; 
distribution of, 98 ; causes of, 
(Hann), 99 ; regions of summer and 
winter, 101 ; heaviest on western 
counties, 101 ; maps, 92 ; Organisa- 
tion, 89 ; amount at different 
latitudes, 98 ; " One inch," of rain 
(note), 102; at different heights 
above ground, 247 ; signs of coming, 
269, 318 ; Bruckner, on cycles of, 
273 ; Koppen on, 274 
Rainy seasons, 100 
Rayleigh, Lord, 36, 178, 206 
Reaumur, thermometer scale, 73 
Red Sea currents, 197 
Redfield, on Gulf Stream, 186 
Reflection, total, cause of mirage, 177 
Refraction, by atmosphere, 33 
Richmann, J., killed by lightning, 

205 
" Roaring Forties " (latitude 40 S.). 

226 
Robinson, Rev. Dr, his Anemometer, 

96, 251 
Rooks, flight of (weather prognostic), 

321 
Rozier, Pilatre de, balloon ascent, 

313 
Ruskin, J., 180 



ST ELMO'S fire, 214 

St Luke's summer, 229 

St Martin's summer, 230 

St Swithin's and St Bartholomew's 
days, 229 

Sargasso Sea, 193 

Schreiber, Dr, wind and barometer, 
66 

Schwabe, Hofrath, discovers perio- 
dicity of sun-spots, 277 

Scirocco, the, of Italy, 156 

Scoresby, Captain, 177 

Scott, Dr R. H.; on measurement of 
solar radiation, 85 ; anemometer 
measurements 98 ; cyclone winds, 
119, 120 ; air pressure and wind, 
151 ; land and sea breezes, 159 ; 
hoar frost, 168 ; heating action of 
Gulf Stream, 187 ; Ocean Currents, 
195 ; thunderstorms, 214 ; cumulo- 
stratus clouds, 271 

Screens (thermometer), Stevenson's, 
82 ; Glaisher's, 83 

Scud (clouds), 138 

Sea and land breezes, 158 ; minimum 
temperature of, 310 ; times of 



maximum and minimum tempera- 
ture, 305 

Seasons, course of, described, 289 ; 
dry and rainy, 100 

Second Order Stations, equipment, 

Secondary cyclones, 107 

Shakespeare, W., on cloud forms, 
145 

Shaw, Dr W. N., 160 

Siberia, winter anticyclone of 
Eastern, 64, 68 

Simoon, the, 128 

Simpson, Dr., theory of thunder- 
storms, 205 

Sky, blue of, supposed cause, 36 

Snow, 164 

Solar radiation, measurement of, 
85 

Southerly bursters, 109, 156 

Sprung, Dr, atmospheric pressure at 
different altitudes, 69 (note) 

Stations, Second Order, 245 

Stewart, Dr R. W., on the mirage, 
177, 178 ; lightning conductors, 
210 

Strato-cumulus, 133 

Stratus cloud, 136 

"Summers," St Luke's, St Martin's, 
Indian, 229, 230 

Sun and atmospheric tides, 64, 183 ; 
constancy of his action, 287 ; 
Herschel on, 288 ; " watery," 263 

Sunshine recorder (Campbell Stokes), 
93 

Sun-spots and magnetic storms, 218; 
periodicity of, Schwabe on, 277 ; 
supposed connection with meteoro- 
logical phenomena, 279 ; and ter- 
restrial magnetism, 278 

Symbols, explanation of, 259 

Symons, J. G., founds British Rainfal 
Organisation, 89 ; Meteorological 
Magazine, 64, 89 ; rainfall and sun- 
spots, 280 



" TABLB - CLOTH " (Cape of Good 
Hope), 144 

Taylor, R. , large hailstone, 166 

Temperature, daily range, 86 ; low/ 
winter, in Eastern Siberia, 68 ; 
mean, 87 ; highest and lowest 
known, 302 ; times of highest and 
lowest, 305 ; at great heights 314 ; 
minimum of sea, 310 

Temple of the Winds (Athens), 260 

Thermic anomaly, 301 



336 



Index 



Thermometer, its invention, 71 ; 
Florentine, 72 ; mercurial, 80 ; 
scales, 73 ; Phillips' maximum, 78 ; 
Negretti and Zambra's maximum, 
78 ; Rutherford's minimum. 79 ; 
Casella's minimum, 253 ; self- 
recording (Thermograph), 81 ; black 
bulb, 84 ; reliability of, 88 

Thompson, Prof. S." P.; theory of 
thunderstorms, 205 ; period of 
magnetic movements, 221 

Thorpe, Sir T. E., on hydrogen, 40 ; 
on ozone, 329 

Thunder, cause of, 208 ; clouds, 211 

Thunderstorms, theories of, 205 ; 
kinds of, 212 ; great violence in 
tropics, 214 

Tides, atmospheric, 64, 183 

Toricelli and the barometer, 46 

Tornadoes, 128 

Torrid zone, 291 

Trade winds, 153 

Tramontana (north wind of Italy), 
156 

Tropics, 28, 292 

Twilight, cause of, 35 ; short in 
tropical regions, 291 

Tyndall Prof. J., on fog formation, 
143 ; on blue of the sky, 178 ; on 
absorption by atmosphere, 312 

"Types of weather," southerly, 231 ; 
westerly, 233; northerly, 233; 
easterly, 234 



VERKHOYANSK, coldest inhabited 
place on the globe, 299, 301 

Vernier for barometer, 51 

Vintage, times of, Briickner's in- 
vestigation, 276, 326 

Volta, theory of hail, 165 ; on atmos- 
pheric electricity, 201 



Von Reichenau. on blossoming times, 

322 
V-shaped depressions, 108 



WALDO, Dr Frank, 92, 244, 275, 322 
Water, great specific heat of, 294 

spouts, 129 

Watery sun, sign of coming cyclone, 

263 

Watson, Dr W., his "Physics," 65 
Weather glass, 62 tt se</. ; signs, 260 

et seq. 
goodness of British, 130 ; 

Reports of Meteorological office, 

Daily, 304, 258 ; Weekly, 259 
Wedges, 110 
Wells, Dr, on dew, 167 
Whiston, Bishop, on a "watery 

comet," 44 
Wilson - Barker, Captain D., his 

cloud classification, 133 ; varieties 

of cumulus clouds, 268 
Wind, instruments for measurement, 

96; "veering" and "backing," 

117 ; direction, inclined to isobars, 

118 ; brave westerly winds of 
Southern Ocean, 69 ; intensity, 
149 ; south - westerly in Europe, 
154 ; north-westerly in Asia, 154 ; 
Beaufort's scale, 149, 159 ; Glaisher 
on Greenwich winds, 155 



YOUNG, Prof. C. A., 218, 281 

, Dr Thomas, the eriometer, 176 



ZAMBRA, see Negretti and Zambra. 
Zodiac, signs and constellations, 25 

(note) 
Zones, 27, 291 



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